1.9 Genetic variation

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1.9 genetic variation

Achievement

Demonstrate

understanding of biological ideas relating to genetic variation. This involves

recognising, naming, drawing, giving characteristics of, or an account of

genetic variation.

Make sure you can

describe:

how DNA carries instructions

to the next generation

the link between DNA and

phenotype

the differences between a

chromosome, DNA and a gene

the relationship between

genes and alleles

the link between genotype and

phenotype

how chromosomes exist in

pairs

how individuals inherit two

copies of most genes

the importance of having

different alleles of a gene

how mutations form new

alleles

how meiosis produces gametes

how sexual reproduction

produces genetic variation between individuals within a group of living

organisms

the possible genotypes, and

phenotype ratios of a simple monohybrid cross showing complete dominance

the possible genotypes, and

phenotype ratios of a simple monohybrid cross showing sex determination

outcomes of a simple

monohybrid cross using a Punnett square

genotype and phenotype

outcomes of a simple monohybrid cross shown in a pedigree chart

inheritable variation that

exists within a group of living things

non-inheritable variation

that exists within a group of living things

how variation in phenotype

within a group of living things gives differing rate of survival

the advantages and

disadvantages of sexual reproduction

describe the specialised

terms outlined in the subject content.

Achievement with Merit

Demonstrate in-depth

understanding of biological ideas relating to genetic variation. This involves

explaining how or why genetic variation occurs.

Make sure you can

explain:

the importance of DNA as the

molecule carrying instructions for life to the next generation

how DNA determines phenotype

the relationship between a

chromosome, DNA and a gene

the importance of having

different alleles of a gene

how genotype determines

phenotype

why chromosomes exist in

pairs - one from each parent

why individuals inherit two

copies of most genes - gives more genetic variation

how alleles of a

gene differ – the order of bases in the DNA

how changes in the DNA

(mutations) may or may not form a new alleles

how meiosis produces gametes

with genetic variation

how meiosis and fertilisation

produce genetic variation between parents and offspring

the reasons for the possible

genotype, and phenotype ratio of a simple monohybrid cross showing

complete dominance

the reasons for the possible

genotypes, and phenotype ratios of a simple monohybrid cross showing sex

determination

how variation in phenotype

within a group of living things gives differing rate of survival

how variation within a group

of living things is important in a changing environment such as pest

infestation, disease, drought or flood

reasons for the advantages

and disadvantages of sexual reproduction

Achievement with Excellence

Demonstrate

comprehensive understanding of biological ideas relating to genetic variation.

This involves

linking biological ideas about genetic variation. It may involve explaining,

elaborating, applying, justifying, relating, evaluating, comparing and

contrasting, or analysing.

Make sure you can:

describe and explain the

ideas listed above for achieved and merit

use linking words and

phrases, such as ‘because’, ‘this means that’, ‘whereas. ‘however;’ when

comparing and contrasting, discussing, elaborating etc.

show you understand that

probability outcomes indicated in Punnett squares are dependent on chance

and effected by sample sizes

demonstrate an understanding

of chromosome numbers and genetic variation related to meiosis and sexual

reproduction.

Make sure you have

up-to-date course notes and don't forget to use them. If you haven't got them

see your teacher.

These are the topics

you may be tested on:

The continuity of life based

on the inheritable nature of DNA. This includes:

the roles of DNA in both

carrying instructions to the next generation and determining phenotype

the relationship between

DNA, alleles, genes, and chromosomes

the way in which genotype

determines phenotype

the way chromosomes exist as

pairs so that individuals inherit two copies of each gene.

links between DNA and

variation in phenotypes. This includes:

the significance of an

allele as an alternative version of a gene

the role of mutations in

forming new alleles

the role of meiosis in

generating gametes (students are not required to provide the names of the

stages of meiosis)

the significance of sexual

reproduction (in producing a new mix of alleles)

the patterns of inheritance

involving simple monohybrid inheritance showing complete dominance, sex

determination, possible genotypes, and phenotype ratios.

variation in phenotypes as

adaptive features. This includes:

inheritable and

non-inheritable variations that exist within a group of living organisms

differing rates of survival

by various members of a group may depend on their phenotype

the importance of variation

within populations (population and species survival) in a changing

environment such as pest infestation, disease, drought, or flood

the advantages and

disadvantages of sexual reproduction.

Know the following genetic

language and conventions:

gene

allele

mutation

genotype

phenotype

gamete

zygote

dominant

recessive

homozygous

heterozygous

pure breeding

Punnett square

pedigree chart

Make sure your course notes

are up to date. If not, see your teacher and use the forum threads and the

rest of the Studyit 'find what you need to know' pages for this standard.

Genetics has a very specific

vocabulary so it is important to use the correct scientific terms in your

answers.

Know these terms:

describe

explain

discuss

compare and

contrast

These are terms that

are used often in science questions. They tell you the way you should write

your answer. From your answers, the assessor will judge your understanding of

biological ideas.

Use the statement 'because'

often, to show your understanding. For example: Both parents must be

heterozygous because some offspring show the recessive phenotype even

though both parents show the dominant phenotype .

Ensure you spell mitosis and

meiosis correctly. If it is not clear which process is being referred to,

your answer may be given a lower grade.

Blue genes: The principles of genetic inheritance

This

programme is designed to teach students about the principles of genetic

inheritance. The parts that are suitable for NCEA level 1 include 'Background

of genetics' and 'Introduction to heredity'.

DNA from the beginning

'DNA

from the beginning' is an animated primer on the basics of DNA, genes and

heredity. A useful resource for upper primary and secondary levels. Some

sections are beyond level 1 so use the index carefully to find sections

relevant for this standard.

Genetic Science Learning Center

This

website from the University of Utah provides information, animated tours, and

interactive activities about genetics. Topics include basic concepts about DNA

and genes, genetic disorders, cloning, stem cells, and more. Some sections are

beyond level 1 so use the index carefully to find sections relevant for this

standard.

Mitosis and meiosis

This

site contains text, an audio component, and an animation which outline the

differences between mitosis and meiosis.

Check out No Brain Too Small for revision notes,

flash cards, testyourself activities, Powerpoints and much more.

Shared

activities for biology

This website has many revision-type activities and games to help you

learn important concepts. Many of the activities have some relevant questions.

When you are not sure if you need to know a particular term or idea you should

check the Studyit content page for this standard.

The

activities relevant to this standard include:

genetics terms matching activity

genetics – hangman

Mendel and heredity

genetics battleship

Allele: Different version of a gene/ alleles are genes that occupy

the same position on homologous (similar)

chromosomes.

Artificial selection: The process of breeding plants and animals

with desirable characteristics in the hope that their offspring will inherit

them.

Asexual reproduction: Reproduction involving one parent producing

identical offspring.

Bases/base pairs: Adenine, guanine, cytosine, thymine.

Carrier: Has inherited a genetic trait or mutation, but does not

display the trait, and can pass it onto their offspring.

Characteristic: An attribute of an individual. E.g. Brown eyes,

tall.

Chromosome: Strands of DNA

which contain many genes; A human cell has 46. (23 pairs)

Complementary base pairing: Use to describe how the bases A always

bond with T, and G always bonds with C.

Continuous variation:

Variation that shows a range E.g. skin colour, height.

Diploid: Cell with the full complement of chromosomes.

Discontinuous variation: Variation that is either/or E.g. Roll

tongue, attached earlobes.

DNA: The chemical which caries genetic information in the nuclei of

cells, made up of phosphate units, sugar units and 4 types of bases.

Dominant: This is the allele which will be expressed in the

phenotype if it is present as either a single gene or two genes.

Double helix: Name given to the twisted ladder shape of DNA.

Fertilization: Male sex cell (gamete) combines with a female sex

cell (gamete) to form a zygote.

Gamete: Sex cells. E.g. sperm and egg.

Gene: Piece of DNA which codes for a particular protein and

therefore a characteristic e.g. tongue rolling.

Genotype: This is what genes you carry e.g. BB, Bb, bb

Haploid: Cell with half the full complement of chromosomes.

Heterozygous: Having a dominant and a recessive allele.

Homozygous: A pair of alleles

that are the same; either both dominant or both recessive E.g. RR or rr

Hydrogen bonds: Attractive forces that holds the two strands of DNA

together.

Incomplete dominance: Situation where neither of the two genes

(alleles) present masks another.

Karyotype: A photograph or diagram of the chromosomes of a cell

arranged in orderly fashion.

Meiosis: This is cell division that reduces the number of

chromosomes to half the normal number, producing 4 genetically different cells.

Mitosis: This is genetically exact division of a cell - Produces 2

identical daughter cells.

Mutation: A sudden, permanent change in a gene or whole chromosome.

Natural selection: Organisms with characteristics best suited to

their environment become more successful, and so expand in numbers.

Pedigree charts: family tree

drawn with standard genetic symbols, showing inheritance patterns for specific

characteristics.

Phenotype ratio: Ratio phenotypes.

Phenotype: The expression of the genotype; this is what you look

like e.g. brown hair, blue eyes, if it's a visible trait.

Punnet square: The name given to the grid of squares that may be

drawn to show he range of combinations of genes that occur.

Pure Breeding: group of

identical individuals at always produce offspring of the same phenotype when

interbred/ are homozygous.

Recessive: his is the allele which is only expressed if there are

two recessive alleles. E.g. bb

Sexual reproduction: Reproduction involving two parents, where the

offspring have some features inherited from each.

Trait: another word for characteristics.

Variation: The differences among parents and their offspring or

among individuals in a population.

Zygote: Cell formed when a sperm cell fuses with an egg cell.

DNA is a

complex chemical that carries genetic information. DNA Is contained in

chromosomes, which are found in the nucleus of most cells. The gene is the unit

of inheritance and different forms of the same genes are called alleles.

Chromosomes

Chromosomes

are x-shaped objects found in the nucleus of most cells. They consist of long

strands of deoxyribonucleic acid, or DNA for short. A section of DNA that has

the genetic code for making a particular protein is called a gene.

The gene

is the unit of inheritance, and each chromosome has several thousand

genes. We inherit particular chromosomes

through the egg of our mother and sperm from our father. The genes on those chromosomes carry the code

that determines our physical characteristics, which are a combination of our

two parents.

The bases

in the DNA molecule carry the different codes needed for different amino acids.

The code for a particular amino acid is made from three bases in a particular

order.

Variation

Variation means

differences between individuals. Variation arises from inheritance,

environmental factors and mutations. There are two types of variation:

Continuous:

In continuous

variation there is a complete range of measurements from one extreme to the

other.

Heightis an example

of continuous variation - individuals can have a complete range ofheights, for

example, 1.6, 1.61, 1.62, 1.625 etc metres high.

Discontinuous:

Variation

that is either/or E.g. Roll tongue, attached earlobes.

Chromosomes:

Chromosomes

are long coiled lengths of DNA found in the nucleus of the cell. Each

chromosomes are made up of many genes. Different species of animals and plants

contain different numbers of chromosomes.

Humans have 46 chromosomes (23 pairs). A pair of chromosomes are called

a homologous pair.

Genes:

Are

sections of DNA that code for particular traits such as eye colour, hair

colour, ear shape.

Alleles:

The name

given to one of the forms that a gene can have. E.g. The gene for tongue

rolling has 2 alleles; a dominant form and a recessive form.

DNA:

A

molecule that contains the instructions to make a new organism It is found in

the nucleus of the cell. It is usually super twisted to form chromosomes.

Chromosomes exist as pairs so that individuals inherit

two copies of each gene.

The

backbone of the DNA molecule consists of two strands of alternating chemical

strands of alternating sugar molecules and phosphate groups, the two strands

twisted to form a double helix. Each sugar molecule is attached to one of the 4

bases called adenine, guanine, thymine and cytosine (A, G, T, C for short) The

bases are paired up on opposite strands. A is always paired with T, and G is

always paired up with C. This is called complementary base pairing - the order

of bases in one strand determine the order of bases in another. A nucleotide is

a basic building block for DNA made up of phosphate, sugar and base. The two

strands are held together by weak hydrogen bonds. A gene consists of hundreds

or thousands of bases. A gene codes for a particular protein by its particular

base sequence.

DNA must

copy or replicate itself before any cell division can take place. The DNA Double Helix is perfectly suited for

replication because each strand can serve as a template to produce a strand

opposite to itself.

First it

is "unzipped" - the two strands are separated. New nucleotides in the

cell line up alongside the unpaired bases, A pairing with T and G pairing with

C. The new nucleotides are joined together using an enzyme called DNA

polymerase.

DNA

replication is semi conservative i.e. each strand in the original DNA molecule

is used as a template o make a new strand of DNA. Each new DNA molecule

contains an original strand and a newly made strand.

Cell division

Cell

division occurs through mitosis and meiosis.

Biological ideas relating to mitosis and meiosis are limited to:

Purpose

Where they occur

Sequence of events (The name

of stages are not required)

Reasons for maintenance or

change of chromosome number

Significance of the number of

cells produced.

Cells

grow old and die. They need to be constantly replaced. This is done by a type

of cell division called mitosis. Mitosis takes place in all cells except the sex organs(testes

and ovaries) t is used for growth and replacement of cells. If you are going to

grow you have to make more cells! Daughter cells have the same number of

chromosomes as the parent cell. Meiosis - Also called reduction division - Is the cell

division that produces gametes. Meiosis in animals takes place in the testes

and ovaries. It results in the production of sex cells (sperm and egg) Sex

cells have half the number of chromosomes as the parent cell. When the sperm

and egg cells unite at fertilization, each contributes 23 chromosomes so the

resulting embryo will have the usual 46.

Meiosis also allows genetic variation through a process of DNA shuffling

(Crossing over) while the cells are dividing. The two types of cell division

are compared below:

Mitosis

Meiosis

In

somatic cells (general body cells.

In

cells producing the gametes (testes and ovaries)

One

cell division, resulting in 2 daughter cells

Two

cell divisions, resulting in 4 cells

Chromosome

number remains the same (diploid)

Chromosome

number halved (haploid)

No

pairing of homologous chromosomes

Homologous

cells pair together before first division

No

"Crossing over"

"Crossing

over" of homologous chromosomes when they pair

(exchanging

genetic information)

Conservative

process: Daughter cells genotypes

Identical

to parental cell's genotype.

Process

produces variation: The genetic makeup includes new

Combinations

of genes not found in parental cell.

Occurs

in many different cell types - important for

Animals

to grow and repair.

Only

occurs in specific cells in the body that will become egg

Cells

or sperms cells (Gametes)

To

understand how the differences between mitosis and meiosis occur, you must

understand the sequence of events that takes place inside the cell for each

type of cell division.

Remember

that the point of mitosis is to produce identical copies of cells for rapid

growth and repair. The point of meiosis is to produce sex cells that contain

half the number of chromosomes. Also during meiosis, genetic information (bits

of chromosomes called chromatids) is swapped between homologous chromosomes

when they line up prior to separation. Mendel formulated what is now known as

Mendel's law of independent assortment. The law states that allele pairs

separate independently during the formulation of gametes. Therefore, Traits

(characteristics) are transmitted to offspring independently of one another. It

is a random process.

Mitosis

Before cell

division, the chromosomes can't be seen. As division starts the

chromosomes become fatter and more visible.

The DNA making each

chromosome copied. Each chromosome has been copied. The copies are still

attached at the centromere. Each copy is called a chromatid.

The chromosomes line up

singularly n the middle of the cell (equator).

One chromatid of each

chromosome goes to the opposite ends of the cell (poles), pulled by the

swindle apparatus. The parent cell starts to split into two.

Two daughter cells are made.

The number of chromosomes are the same in each daughter cell as the

original parent cell.

Meiosis

Chromosomes

become fatter and more visible.

The DNA making each

chromosome is copied. Each chromosome has been copied. The copies are

still attached at the centromere. Each copy is called a chromatid. The

homologous pairs of chromosomes line up along side each other in the

middle of the cell. Now some crossing over of bits of chromosomes can take

place between homologous pairs. This swaps genes from one chromosome to

another and leads to variation between offspring.

One of each homologous pair

goes to opposite ends of the cell. The parent cell starts to split into

two, making two daughter cells.

The chromosomes in each cell

line up in the middle and this time the chromatids separate. We now have 4

daughter cells; each one has half the number of chromosomes are the

original parent cell. Also, these chromosomes are not identical to the

parent because crossing over took place and independent assortment

occurred.

Mutations - When things go wrong with DNA.

It is

important that DNA remains unchanged rom generation to generation. The

complementary base pairing that occurs during DNA replication means this

generally occurs. However, random, spontaneous mistakes can happen.

A

mutation is a change in the type

or the amount in the DNA in a

cell.

Type of DNA: A mistake made in copying the DNA

can produce a slightly different allele of a gene. Certain chemicals,

ultra-violent rays, or radiation can increase the chances of changes occurring

to DNA even when chromosomes are not being copied.

Changes

to the base sequence are called point or gene mutations and can be (a)

insertion (addition of a base) (b) deletion (loss of a base) c inversion (2

bases change position) (d) substitution (one base replaces another). If the

mutation happens in the gametes, then this allele can be passed onto offspring

- these diseases are called inherited diseases eg cystic fibrosis.

Mutations

can be:

Harmful (causing death or

disease): To function correctly, cells depend on many proteins. Gene

mutations may stop one or more proteins from working properly. By changing

a gene's instructions for a protein, the mutation can cause the protein to

malfunction or to be missing entirely. If the protein plays a critical

role in the body, it can disrupt development or cause a medical condition.

Some mutations can cause illnesses such as cancer.

Beneficial( give some

advantage or benefit to an organism): eg antibiotic resistance in bacteria

is beneficial to the bacteria.

Silent/neutral - Appear to

not affect the organism (does not affect the phenotype).they may not be

evident if the mutation is a recessive allele.A change to a DNA triplet

might not change which amino acid is introduced, or it might of change the

amino acid to a chemically similar amino acid that works just as well.

Silent/neutral mutations do however create the possibility of a future

mutation having an effect.

Growth and repair

Produces two genetically

identical cells

Cells are diploid (full

chromosome set), 2n

Occur in somatic cells

New cells

are needed throughout life - we do not have the same cells all throughout our

life. These are for growth, to replace damaged cells and repair worn out

tissues. Normal human body cells are diploid - they have two of each

chromosome(they have a full set of chromosomes) . When new cells are made,

these 46 chromosomes are copied exactly in a process called mitosis. Mitosis is the type of cell division used for

growth and repair. Mitosis occurs whenever new cells are needed. It produces

two cells that are identical to each other and the parent cell.

In

mitosis, each chromosome is copied exactly. The new chromosomes are moved to

opposite sides of the cell, before the cell divides leaving one complete set of

46 chromosomes in each of the two new cells.

Constant

cell division ensures that cells never get two large. The larger the cell

becomes, the smaller its surface area to volume ratio. Objects with a small

surface area to volume ratio find it difficult to maintain exchange of

materials with their enviroment. Large cells could run out of oxygen, and

accumulate too much waste, such as carbon dioxide. For this reason it's more

efficiant for large organisms to be multicellular, rather than single celled.

Fertilization

In humans, all reproduction is sexual. It

involves joining together cells from each parent with half the normal number of

chromosomes to make a new cell containing both parents genetic material.

The cells

from each parent that combine to form the zygote are called gametes. In humans,

the male gamete is called sperm, and the female gamete is called an egg. When

the gametes join they form a cell called a zygote. Human sperm and egg contain

23 chromosomes. Human zygotes contain 46 chromosomes.

The type

of cell division that produces gametes with half the normal chromosomes is

called meosis.

Gametes

contain different genetic information to each other and to the parent cell.

Meiosis

is responsible for genetic variation.

These are

helpful terms to learn:

Gamete - cell with half the

normal number of chromosomes, and only used for sexual reproduction.

Zygote - cell formed when the

two gametes combine.

Fertilisation - term to

describe the joining of two gametes

Haploid - having half the

normal number of chromosomes.

Diploid - having the normal

number of chromosomes.

Gametes

All

Gametes are haploid. They also have other adaptations to increase the chances

of fertilisation and successful development of the embryo.

Sperm

cells need to move the egg cell. They have a tail to propel them, and many

mitochondria to provide energy. The front of the sperm contains enzymes to

digest the egg membrane.

The egg

contains a large food store to support the developing zygote until it can get

food via the placenta.

Meiosis

Meiosis

is the type of cell diviosion used in sexual reproduction. It occurs only in

the testes and ovaries. It produces cells that are different to each other, and

to the parent cell. The cells produced contain half the normal number of

chromosomes.

At fertilisation, the nuclei of the sperm

and an egg join to form the zygote. The zygote contains 23 pairs of chromosomes - 23 single

chromosomes from the sperm, and 23 single chromosomes from the egg, thereby

creating the correct number of 46 chromosomes for all body cells. It also means

the zygote contains a complete set of chromosomes from each parent.

Amount of DNA: Sometimes when cells divide,

the wrong number of chromosomes ends up in a gamete, and it may have a

particular chromosome missing, or two copies of a chromosome.

How DNA “codes” for proteins

Gene is part of the DNA

molecule/chromosome.

Is a sequence of

bases/nucleotoids.

Codes for a specific sequence

of amino acids

MITOSIS AND MEIOSIS

1.9 GENETIC VARIATION

SUBJECT CONTENT

KEY STUDY TIPS

TERMS GLOSSARY

DNA, CHROMOSOMES, CELL DIVISION