Linkage, Recombination and Ratios Flashcards
What are sources of genetic variability?
Mutation
Independent assortment of chromosomes during meiosis I
Crossing-over between homologous chromosomes during meiosis I
Random fertilization of ova by sperm
With 23 pairs of chromosomes, how many possible combinations of chromosomes types?
2^23
How many combinations of offspring from one couple are there?
2^46
What is a recombinant phenotype?
Is one where the combination of phenotypes differs from that found in either of the parents
What is crossing over?
The process of genetic recombination that gives rise to new combinations of linked genes
When does crossing over occur?
Pachytene phase of prophase I
How does crossing over occur?
Begins with synapsis. The synaptonemal complex is a protein ‘zipper’ that holds homologous chromosomes together in the tetrad. Formation of the synaptonemal complex leads to crossing over between homologous chromosomes.
What is the result of crossing over?
Recombinant chromosomes with new combinations of linked genes
What is recombinant frequency?
The percentage of the progeny that inherit a combination of alleles that differs from either parent
What is studied to estimate recombination frequencies?
A testcross
Who was the first to observe gene linkage?
TH Morgan
Define incomplete dominance
Dominant allele does not completely mask the trait ef the recessive allele eg, pink red and white
Define co dominance
Where each allele affects phenotype in separate distinguishing ways eg, I^A, I^B, I^AB
Define pleiotropy with examples
Where a single gene has multiple effects on the phenotype eg. gene for flower colour also affecting seed colour
Define polygenic inheritance with examples
Where a single trait is determined by multiple genes eg. human height, weight, skin pigment
