Flashcards

3.7.1 Inheritance

megan trent English 58 cards

Cards in this deck (58)

1In 1886 Gregor Mendel suggested thatthe characteristics of organisms were determined by ‘units’ which were handed on from generation to generation. These ‘units’ have been identified as genes which were carried on and transmitted by chromosomes.
2Genes have 3 main characteristics:They can separate and combine
They can mutate
They code for the production of specific polypeptides
3One gene is the portion of DNA bases on a chromosome whichcodes for one polypeptide. Alleles are alternative forms of genes occupying a similar ‘gene position’ or Locus on homologous chromosomes.
4Gene -A sequence of bases on a DNA molecule that codes for a protein (polypeptide) which results in a characteristic, e.g. a gene for eye colour
5Allele –A different version of a gene. There can be many different alleles of a single gene but most plants and animals, including humans, only carry two alleles of each gene, one from each parent. The order of bases in each allele is slightly different. They code for different versions of the same characteristic. Only one allele of a gene can occur at the locus of any one chromosome.
6Genotype-the genetic constitution of an organism, the combination of alleles found in an individual
7Phenotype-the appearance of an organism determined by its Genotype
8Dominant allele -The allele that is always expressed in the organism’s Phenotype e.g. T for tall
9Recessive allele-The allele that is only expressed in the phenotype when present as a pair of recessive alleles (homozygous recessive) e.g. t for short
10Codominant –Alleles that are both expressed in the phenotype – neither one is recessive
11Locus –The fixed position of a gene on a chromosome. Alleles of genes are found at the same locus on each chromosome in a pair
12Homozygous-if both alleles are the same e.g. TT or tt
13Heterozygous-if alleles are different e.g. Tt
14Carrier –A person carrying an allele which is not expressed in the phenotype but that can be passed on to offspring
15Gene expression -the transcription and translation of a gene, resulting in a functional protein which is part of an individuals phenotype
16F1-First generation
17F2Second generation
18Crossing over – prophase 1 leads to variation
19Random AssortmentRandom assortment of homologous pairs of chromosomes at Metaphase 1 gives new combinations of chromosomes
20Random Assortment diagram
21Random fusion of gametes at fertilisationRandom fertilisation in which any possible gamete from the male can randomly fertilise any other possible gamete from the female.
The ovum has 8 million (2^23) possible chromosome combinations, so does the sperm cell. 8 million x 8 million = 64 trillion possible diploid combinations in each and every zygote.
Therefore meiosis and random fusion of gametes gives rise to tremendous variety
22GenomeThe total genetic make up of an organism.
23Monohybrid inheritanceInvolves the inheritance of a single characteristic (2 alleles involving a single gene)
24Dihybrid inheritanceInvolves the inheritance of two separate genes.
25CodominanceWhere the heterozygous individual has a phenotype intermediate between the two homozygous parents.
26Linkage –Refers to two or more genes that are found on the same chromosome. Linked genes are inherited together.
27AutosomeA chromosome that is not a sex chromosome.
28Sex linkage –A characteristic is sex linked if the gene that controls it is on one of the sex chromosomes
29homologous chromosomes, genes, and alleles
30Genetic terms
31At metaphase alleles are randomly arranged on the equator and will be distributed randomly into gametes
32Monohybrid InheritanceInvolves the inheritance of a single characteristic
33Monohybrid Inheritance Mendel’s experimental organism Why peas?Variety of contrasting traits (controlled by single genes that behave independently)
Shape of pea flower protects it from foreign pollen (self pollinator)
Pea plants grow quickly and don’t require much space.
34Traits that Mendel studied
35As a result of the early experiments Mendel formulated his first law of inheritance'The characteristics of an organism are determined by factors (alleles) which occur in pairs.
Only one of a pair of factors (alleles) can be present in a single gamete‘
Known as the law of segregation
36Monohybrid crossesMendel’s 1st law of inheritance “The characteristics of an organism are determined by factors (alleles) which occur in pairs. Only one of the pair of factors (alleles) can be present in a single gamete” This is also known of as the law of segregation
The inheritance of one gene (two alleles) from a homologous pair of chromosomes is called monohybrid inheritance.
Capital letters represent dominant alleles e.g. R
Lower case letters represent recessive alleles e.g. r
RR = homozygous dominant
Rr = heterozygous
rr = homozygous recessive
Let R = dominant allele for Round peas
Let r = recessive allele for wrinkled peas
37Monohybrid Crosses – Set out genetic crosses in the following way Cross 1: Cross pollinate homozygous round pea plants with homozygous wrinkled pea plants (pure breeding)
38Monohybrid Crosses – Set out genetic crosses in the following wayCross 2: Cross pollinate/interbreed heterozygous round pea plants (F1)
39Monohybrid crossesAt fertilisation any male gamete can fertilise any female gamete at random. Two heterozygous individuals always produce offspring with a phenotypic ratio of three dominant to one recessive.
40Notice gametes areinside circles and you only write down the gametes from a parent that have different genotypes.
41Monohybrid Crosses – Set out genetic crosses in the following wayCross 2: Cross 3 (Test cross): Useful cross to find out if an individual with the dominant characteristic/phenotype e.g. Round pea, is homozygous RR or heterozygous Rr:
42You can see an individualsphenotype but not its genotype
43A testcross can be used todetermine if a particular dominant characteristic observed in an organism is determined by one or two dominant alleles ie in the example below; FF or Ff
The backcross consists of crossing the unknown plant with a known ‘recessive’ genotype. If the resulting offspring are
a mixture of forms then
the unknown parent was heterozygous.
44Co-dominanceNot all characteristics are controlled by a single gene which behaves independently as observed by Mendel
In co-dominance both alleles are dominant, that is the alleles express themselves equally in the phenotype
neither are dominant. To show these crosses, there is a different way of showing the alleles:
There is a capital letter for the Characteristic
Capital letters for alleles are placed superscript on the characteristic letter
E.g. Colour of petals in carnations – red, white, pink
Let C = colour
Let C^R= red allele
Let C^W= white allele
45Co-dominance Cross 1: Cross pollinate red flower plant with white flower plant
46Co-dominance Cross 2: Cross pollinate pink with pink flower plant (F1 hybrids)
47Sex Determination
48Inheritance of SexOne pair of the homologous pairs of chromosomes are the sex chromosomes which carry the genes coding for male and female sexual characteristics. The non sex chromosomes are called autosomes.
49Humans 23 homologous chromosomes, the first 22 areautosomes the 23rd pair are the sex chromosomes:
X = female sexual characteristics
Y = male sexual characteristics
50Females areXX (homogametic) and produce ova that always have X chromosome present.
51Males areXY (heterogametic) i.e. sperm produced can either have X or Y chromosomes:
52Sex LinkageSome alleles are carried on the sex chromosomes and are said to be sex linked
Genes present on the same chromosome are said to be linked and are inherited together.
When crossing over occurs linked genes may get separated to give rise to new combinations of alleles.
53on the Y chromosomeThe y chromosome is much smaller than the x chromosome and carries very few alleles
54MalesAny recessive alleles carried on the x chromosome will express themselves in the phenotype.
This is because they are unpaired and so there is no dominant gene present.
55Sex Linkage Cross 1: Cross male and female
56Punnett SquareA carrier is an individual with one normal allele and one potentially harmful recessive allele.
Notice the way that this is written. H is the allele for normal blood clotting.
57Dihybrid CrossesMendel’s 2nd law of inheritance “Each member of an allele pair may combine randomly with either of another pair”
The inheritance of two genes (four alleles) from different homologous pairs of chromosomes is called dihybrid inheritance.
At meiosis the pairs of homologous chromosomes are separated, one of each pair into one of the two gametes formed. But there is RANDOM/INDEPENDENT ASSORTMENT of chromosomes so all the allele combinations are possible:
58Dihybrid Crosses posible gametes Aa Bb