Flashcards

Mendelian Genetics

megan trent English 45 cards

Cards in this deck (45)

1why do we study genetics?understanding the nature of hereditary mechanisms allows you to
direct evolution, cherry picking desirable traits
2history of gregor mendelborn in 1822
1856 – began pea experiments
1865 – presented findings to the Brunn Society of
Natural Science
1866 – published pea experiments
3Mendel’s Theory of InheritancePostulated a theory of particulate inheritance
with discrete factors acting within specific
rules
– Transmission genetics
– Garden pea (Pisum sativum)
Is now widely regarded as the father of
genetics
4Mendel’s laws First Law : The Law of Segregation -- during gamete
formation each member of the allelic pair separates from the
other member to form the genetic constitution of the gamete
(To understand this he kept pure lines, meticulous notes and
used statistical analysis to understand what was significant and
what was not)
5Mendel’s laws Mendel's Second Law: The Law of Independent Assortment -during gamete formation the segregation of the alleles of one
allelic pair is independent of the segregation of the alleles of
another allelic pair
This is exemplified during a di-hybrid cross
6Mendel’s Theory of Inheritance” Mendel began counting the
number of wrinkled and round peas in the F2 generation in relation to the F1
All things being equal we would predict that there would be 50% round and
50% wrinkled.
However this prediction did not match the observed data.
7Mendel’s Theory of Inheritance Pea experiment overviewCrossed pure breeding lines
i.e. rr x rr or RR x RR
Seeds were grown to produce the first
filial generation (F1)
F1 seeds were crossed to produce the
second filial generation (F2)
Pure-breeding: something that
will pass the same phenotype
down through the generations
Filial generation: A generation in a breeding experiment that is successive to a mating
between parents of two distinctively different but usually relatively pure genotypes
8Mendel’s Theory of Inheritance Pea Experiment diagram
9Mendel’s Theory of Inheritance
10What Happens If We Cross Heterozygous (Rr) Parents with round seeds?
11Punnett SquareReginald C. Punnett (1900s)
• Used to determine/predict the phenotypic outcome for crosses between known genotypes
12example of punnet squre
13Homozygous (Recessive) Vs Hetrozygous
14Homozygous (Dominant) Vs Hetrozygous
15Hetrozygous Vs Hetrozygous
16Demonstrating Reproducibility and the Robust Nature of This Theory:This information can be used to confirm genotype, if the
genotype of one of the parents is known.
? Vs. Rr
If ? = RR you get a ratio of 1:0
If ? = Rr you get a ratio of 1:3
If ? = rr you get a ratio of 1:1
17The Test CrossZygocity is the degree of similarity of alleles for a trait in an organism.
This is asking is its genotype homozygous or heterozygous ?
This is where an unknown individual is crossed with a
phenotypically recessive partner (rr) to assess the zygocity
(genotype) of the unknown
If ? = RR you get a ratio of 1:0
If ? = Rr you get a ratio of 1:3
If ? = rr you get a ratio of 0:1
18Gene:a unit of heredity which is transferred from a parent to offspring and is held to
determine some characteristic of the offspring
19Locus:a particular position or place where something occurs or is situated
20Allele:One of the different forms of a gene that can exist at a single locus
21Homozygous:An individual with 2 identical alleles i.e. RR or rr
22Heterozygous:An individual with 2 different alleles i.e. Rr or rR
23Genotype:A description of the genetic composition of an organism(i.e. Rr, RR or rr).
Genotypes tells us which alleles an individual has
24Phenotype:The observable characteristics displayed by an organism. This is (generally)
the manifestation of the genotype
25Ploidy numberThe number of copies of a gene or sets of chromosomes in an organism
26Summary: Mendel’s Principles of Inheritance Unit factors in pairsCharacters were controlled by factors which carried the information generation to generation. Each individual carries 2 factors (1 from each parent) for any
characteristic (genes)
27Summary: Mendel’s Principles of Inheritance Dominant/recessive characteristicsWhen two unlike factors responsible for a single character are present in a single
individual, one unit factor is dominant to the other which is said to be recessive
28Summary: Mendel’s Principles of Inheritance Principle of segregationDuring the formation of gametes, the paired unit factors separate or segregate
randomly so that each gamete receives one or the other with equal likelihood
29Summary: Mendel’s Principles of Inheritance Principle of independent assortmentThe factors assort independently from each other, whether a pea is round is
determined independently of whether it is yellow
30What Happens When Two Genes Control the Phenotype ?This is done using a dihybrid cross, a cross between 2 individuals
Expressing two different traits controlled by two sets of genes
For this specific example we will consider genes A and B
31What Happens During A Heterozygous Dihybred Cross?
32What Happens in a Homozygous Dihybred Cross?
33What Happens in a Dihybred Test-Cross?
34A backcross of the F1 to the double recessive parent (a testcross) yields a 1:1:1:1 ratio
35Dihybrid testcross
36Mendelian Ratios in Summary• Monohybrid cross
– 1 gene, 2 alleles, clear dominance à 3:1
• Dihybrid cross
– 2 genes, 2 alleles each, clear dominance à 9:3:3:1
• A test cross can be used to assess “zygocity”
assuming:
1. Each allele is dominant or recessive
2. Genes assort independently
37Chromosomal Inheritance1879 Flemming used new dyes to stain cells
saw threadlike structures during cell division
1902 Cytologists Walter Sutton and Theodor
Boveri examining these threads,
(chromosomes), during meiosis, linked their
discoveries to Mendelian principles of
segregation and independent assortment.
They propose the chromosomal theory of
inheritance
38Chromosomal Inheritance Interphase:G (growth)1
S (DNA synthesis)
G2
39Chromosomal Inheritance Prophase:Leptotene (thin threads) - Homologous
Zygotene (paired threads) -recombination occurs
Pachytene (thick threads)
Diplotene (two threads) - Chromosomes become visible under the microscope
Diakinesis (moving through) - Chromosomes become
visible under the microscope
40Chromosomes:Every human, non-cancerous, somatic cell contains 2 copies of each
chromosome
The chromosome complement (Karyotype) is transmitted from parents to
offspring
During meiosis homologous chromosomes pair and then separate to
different gametes (meiosis = a reduction in the cells ploidy number to
haploid)
Maternal and paternal copies of each chromosome separate to each pole
without regard to the assortment of any other homologous chromosomes
At fertilization an egg’s set of chromosomes unites randomly with a
sperm’s set of chromosomes resulting in a diploid cell
In all the cells derived from the fertilized egg half the chromosomes are of
maternal origin and half of paternal origin
41Mendel’s factors Chromosomes
42GenesMendel’s factors are genes, genes are on chromosomes
A gene is a DNA segment that encodes an RNA or protein molecule
Different factors for the same trait are called alleles
Round and wrinkled are both alleles of the wrinkled gene
Genes are in the same place (locus) on both of the homologous chromosomes
Homologous chromosomes can have different alleles i.e. R maternal chromosome, r
paternal chromosome
43The Flow of Genetic Information
44Evolution: Darwin and MendelDarwin 1809-1882 proposed that evolution
requires the selection of specific traits (from a
gene pool) in response to a selection pressure.
This brings about cumulative change within a
population over time. Mutations can enhance
the diversity of a gene pool in both a positive
and negative way.
Darwin didn’t understand how traits and
mutations could be passed form parent to
offspring as at the time it was believed traits
from the mother and father “blended”.
Mendel 1822-1884 provided an explanation as
to how mutations and selected traits could be
passed though the generations.
On the Origin of Species: Published 1859
– before Mendel’s pea experiments
45—