Spring 2009: Section 4 – Lecture 1 Reading: Chapter 2

Mendel’s two laws: The Law of Segregation The Law of Independent Assortment

Law of Segregation - A pair of alleles for a given gene (trait) separate or segregate in the gametes equally. In meiosis this relates to the separation of the homologous chromosome pairs in anaphase I.

Law of independent assortment - Allelic pairs of genes for two traits will behave independently of each other (unless they are close to each other on the same chromosome). In meiosis this relates to the sorting of non-homologous chromosomes in the reductional division.

Abbreviations for alleles dominant allele  capital letter or ‘+’ recessive allele  lower case letter example : RR - homozygous dominant Rr - heterozygous rr - homozygous recessive

Mendel’s laws Law of segregation - A pair of alleles for a given gene (trait) separate or segregate in the gametes equally. example: seed shape in peas Two phenotypes - smooth and wrinkled

Gametes possible in the F1 generation: S and s Way to determine phenotypic and genotypic ratios – Punnett square

F2 phenotypic ratio 3 - S_  since S is dominant it does not matter what allele is here. 1 - ss

F2 genotypic ratio 3 genotypes possible 1 - SS smooth homozygous 2 - Ss smooth heterozygous 1 - ss wrinkled homozygous

How to test for the genotype of an individual: 1. testcross - cross F2 individuals to a homozygous recessive individual. Because one parent is homozygous recessive any variation in the phenotype of the progeny will be due to the other, unknown genotype, parent.

SS x ss Ss x ss ss x ss    100 % - Ss 50% - Ss 100% - ss 50% - ss So if you observe segregation in a testcross then the unknown genotype is heterozygous.

2. If possible, self the individual. SS Ss ss X X X    all SS 3 smooth all ss smooth 1 wrinkled wrinkled So if the progeny segregate then the unknown genotype was heterozygous.

Pedigree analysis If a species has few progeny per year, few progeny in a lifetime, and/or long durations between generations, it can be difficult to get enough progeny to do genetic analysis of a trait.

A way around this problem is to do pedigree analysis of the family, looking back several generations. Symbols male  female  mating 

example: I parents II children

Law of independent assortment - Allelic pairs of genes for two traits will behave independently of each other (unless they are close to each other on the same chromosome).

Example: pea seed shape and color shape: smooth and wrinkled color: yellow and green

F2 Phenotypic ratios 9 - smooth, yellow: S_Y_ 3 - wrinkled, yellow: ssY_ 3 - smooth, green: S_yy 1 - wrinkled, green: ssyy

F2 Genotypic ratios smooth, yellow: 1 SSYY 2 SSYy 2 SsYY 4 SsYy wrinkled, yellow: 1 ssYY 2 ssYy smooth, green: 1 SSyy 2 Ssyy wrinkled, green: 1 ssyy

Three gene example: seed shape, seed color and plant height

Another method to determine phenotypic and genotypic ratios is to use the split fork method.

Sometimes in an experiment you do not get a perfect segregation ratio. If you have a ratio of 2.23 - 1, is it close enough to a 3 - 1 ratio to say you are observing a single gene trait under control of a completely dominant allele?

To aid in your decision you can use probability and statistical analysis to determine if the observed ratio is close enough to the expected ratio. Probability and Statistical Testing Probability - Two basic rules, the product rule and the sum rule

product rule - multiply the probabilities together of two independent events to determine the probability of two independent events will occur together. sum rule - add the probability of two events together when the events are mutually exclusive events.