Linkage, Recombination and Deviations from Mendelian Ratios

What are the four processes that lead to genetic variation?

mutation / independent assortment / crossing over / random fertilisation

How does random fertilisation lead to genetic variation?

the sperm that fuses with the egg is random

the egg that fuses with the sperm is random

What is a recombinant phenotype?

Where the combination of phenotypes differs from that found in either of the parents

Describe crossing over

occurs during the pachytene phase of prophase 1 / begins with synapsis / the synaptonemal complex holds together homologous pairs in a tetrad / genes will swap over from homologous pairs due to the formation of the complex / leads to recombinant chromosomes

what is recombination frequency?

the percentage of progeny that inherit a combination of alleles that differs from either parent

what is the equation for recombination frequency

number of recombinants / total number of progeny x 100

how are recombinant frequencies estimated?

using testcrosses

what would be the expected result of a test cross using genes that are on different chromosomes?

50% parental 50% recombinant

what would be the expected results of a testcross using genes that are on the same chromosome?

>50% parental and <50% recombinant

What species did T.H.Morgan experiment on when he discovered linked genes?

drosophila

what is a coupling heterozygote?

the dominant and recessive alleles are on separate homologues

what is a repulsion heterozygote?

the dominant and recessive alleles are on the same homologue

What will be the largest phenotypic class in progeny?

parental classes

What are recombination frequencies related to?

the distance between the two genes on the chromsomes+

why don't frequencies add up to the total distance between two genes?

the distances don't account for multiple cross overs in the genes

What is sex linkage?

the phenotypic expression of an allele that is dependent on the gender of the individual and is directly tied to the sex chromosomes

What is incomplete dominance?

when one allele is not completely dominant over another leading to the blending of traits

what is an example of incomplete dominance in humans?

familial hypercholesterolaemia

What is co-dominance?

each allele affects the phenotype in separate distinguishable ways

What is pleiotropy?

a single genes has multiple affects on a phenotype

What is polygenic inheritance?

a single trait is determined by multiple genes

What is epistasis?

one gene masks or modified the expression of another gene

give an example of epistasis

Allele A is dominant for black fur. Allele B is recessive for brown fur. Gene C determines if the colour is deposited into the fur or not

what type of mendelian deviation leads to a bell curve of a trait in a population?

polygenic inheritance

how many possible chromosome combinations are there for an offspring?

2^46

how is the normal (wild-type) allele written?

with a superscript +

what kind of gene arrangement do coupling and repulsion heterozygote come from

double heterozygote