Chapter 5 Genetics

Unlinked Genes

assort independently during Meiosis I

• genes on separate chromosomes

Independent Assortment

• gametes will be evenly distributed

• gametes combos possible in = proportions

• F2 offspring show a 9:3:3:1 ratio

• 1:1:1:1 ratio of gametes

Linked Genes

genes stick together and typically end up in the same gamete cell

• genes on same chromosome

Unlinked Genes Phenotypic Ratio from Testcross

1:1:1:1 ratio

Linked Genes Phenotypic Ratio from Testcross

only expect to see 2 groups (1:1)

Parental Groups

make up the largest groups

• will tell what P gen looked like

Parental Types

offspring matching parental phenotypes

Recombinants

offspring with non-parental phenotypes

• never account for more than 50% of progeny

Crossing Over

homologous chromatids exchange genetic material during prophase 1 of meiosis

• usually only ½ of chromatids recombine

• 1 parental and 1 recombinant per chromosome

Recombination frequency of 50%

resemble unlinked genes (1:1:1:1 ratio)

NCO

no crossing over

SCO

single crossover

NCO > SCO

number of non-crossover offspring is greater than single crossover offspring

More distance between genes

• higher probability of crossing over between them

• higher frequency of recombination

Genes close together

• less chance of crossing over

• lower recombination frequency

Calculating distance between 2 genes

1. identify parental and recombinant groups

2. calculate recombination frequency

3. draw linkage map

Recombination frequency

(number of recombinants/ total progeny)x100

NCO>SCO>DCO

DCO have the least frequency of offspring

DCOs (Double crossovers)

middle gene gets exchanged, outer genes paired like parentals

Calculating linkage for trihybrid cross/DCO quick way

1. determine gene order from DCOs (smallest groups)

2. identify recombinants for only 2 gene pairs

3. calculate recombination frequency

IF calculating linkage for outer genes

1. identify recombinants for 2 outer genes

2. calculate recombination frequency (add DCOs twice)

3. add together the “bottom” 2 frequencies to get “top” frequency

Males X chromosome

comes from mom (explains why males in F1 gen look like the mother)

Parentals and Recombinants come in___

pairs

RF/2

gives you recombination frequency for each group of a pair

Interference

where the occurrence of one crossover event on a chromosome reduces the likelihood of another crossover happening nearby

• fewer double crossovers are observed than expected

Calculating interference

1. calculate coefficient of coincidence

2. interference= 1-coffecient of coincidence

3. interference is 0 when observed DCO=expected DCO

4. interference is 1 if no DCO occurred

Coefficient of coincidence

observed double crossovers/ expected double crossovers

Chi-square test

test whether genes are actually linked

Chi-square test formula

X² = ∑(o-E)²/E

Null hypothesis

genes are NOT linked

• observed ratios do not significantly deviate from expected value

Degree of Freedoms

number of groups - 1 (typically 3 since 2 point crosses have 4 groups)