Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
AHL Genetics I. Dihybrid Crosses • Monohybrid Aa genotype – one gene / trait • Dihybrid AaBb genotype – two genes / traits • Recombinant – the reassortment of genes or traits into combinations different from those of the parents • Dihybrid cross parents that are both heterozygous. – Example: Mendel’s peas • Yellow vs. green & Tall vs. short – So we are crossing two plants heterozygous for both traits, Yellow and Tall: YyTt X YyTt 1 I. Dihybrid Crosses YT Yt yT yt YT YYTT YYTt YyTT YyTt Yt YYTt YYtt YyTt Yytt yT YyTT YyTt yyTT yyTt yt YyTt yyTt yytt Yytt Dihybrid Cross – Genotypes? YT Yt yT yt YT YYTT YYTt YyTT YyTt Yt YYTt YYtt YyTt Yytt yT YyTT YyTt yyTT yyTt yt YyTt Yytt yyTt yytt Recombinants? Genotypes: YYTT- 1 YYTt - 2 YYtt - 1 YyTT- 2 YyTt - 4 Yytt - 2 yyTT - 1 yyTt - 2 yytt - 1 2 9:3:3:1 Phenotypes YT Yt yT yt YT YYTT YYTt YyTT YyTt Yt YYTt YYtt YyTt Yytt yT YyTT YyTt yyTT yyTt yt YyTt yyTt yytt Yytt Dihybrid cross • Know this ratio! 9:3:3:1 – – – – Yellow Tall Yellow short green Tall green short • Can give other ratios IF… – – – – Either parent is homozygous for one or both traits Either gene has codominant alleles Either gene is sex-linked There is an interaction between the two genes 3 Dihybrid cross Try this! In rabbits, black is due to a dominant gene (B) and brown is due to a recessive allele (b); short hair is a dominant gene (S) while long hair is recessive (s). A black, long-haired rabbit (whose mother was brown), was crossed with a brown rabbit with short hair (whose father had long hair). A black, long-haired rabbit (whose mother was brown), was crossed with a brown rabbit with short hair (whose father had long hair). 1. Key: B – black, b – brown S – short, s – long 2. Parents Phenotypes: Black long, brown Short 3. Parents Genotypes: Bbss, bbSs 4. Gametes: Bs, bs bS, bs 5. Punnett Square: Bs bs bS BbSs bbSs bs Bbss bbss 4 Key: B – black, b – brown S – short, s – long 6. Offspring genotypes: 25% each: BbSs, bbSs, Bbss, bbss 7. Offspring Phenotypes: 25% each: Black Short, brown Short, Black long, brown long 8. Any Recombinants? Yes: BbSs and bbss Bbss X bbSs Bs bs bS BbSs bbSs bs Bbss bbss II. Autosomes & Sex Chromosomes • Autosomes: chromosomes 1-22. • Sex chromosomes: X and Y – they determine the sex of the individual – XX female & XY male 5 III. Linkage Group Definition – Linkage group – all the genes on a chromosome. – Crossing over between non-sister chromatids of a homologous pair in prophase 1 can result in random orientation in metaphase 1 B. Linked Genes • Let’s compare 2 examples: • UNLINKED: If A and B are linked, we write, AB ab – AaBb X aabb ab AB Ab aB ab AaBb Aabb aaBb aabb • Recombinants: – Aabb & aaBb ab ab X ab AB AB ab ab ab ab Recombinants: No! 6 Linked Genes • If A & B linked AB ab ab ab X ab ab AB AB ab ab ab ab • What if you breed them and actually get the following? • 37% AB • 37% ab ab • 13% Ab ab • 13% aB ab • So, 50% and 50% AB ab ab ab Linked Genes • There must have been crossing over! • The probability of crossing over increases, the further apart the genes are on the chromosome. • This used to be used to map chromosomes (the old way). 7 Linked genes • Try these: 1. 2. Ab aB Ab aB (What are the recombinants?) X X ab ab Ab aB All recombinants 50% recombinants IV. Polygenic Inheritance • More than one gene codes for a trait • Example – height • Continuous variation • The environment also is part of variation: good nutrition or poor nutrition. 8 Polygenic Inheritance • Example: skin color • Up to 7 genes (each “dark” codominant allele contributes a certain amount of melanin) • Continuous variation • The environment also contributes melanin (sun causes skin to make extra melanin) Polygenic Traits: skin • UV light – Causes skin to make Vitamin D strong bones – May cause sunburn skin cancer – Destroys folic acid neural tube defects which can kill fetuses • Melanin blocks UV light 9 Polygenic Inheritance • Example: skin color • Natural Selection: – Dark skin has an advantage in direct sun (equator) – Light skin has an advantage further from the equator – Original dark-skinned people travelled north where light skin was selected for – In southern India, people have dark skin, but came from lighter skinned people – Color change in as little as 2500 yrs. Skin color mirrors UV radiation levels That’s it! 10