Introduction to Genetics

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or different forms of genes Page 2 of 11 2 Types of Alleles 1. Dominant allele of a gene that masks or suppresses the expression of an alternate allele - represented by uppercase letter 2. Recessive - allele that is masked by a dominant allele - represented by lowercase letter Genotype - genetic makeup of an organism written in letter form (ex. TT) Phenotype - physical appearance of an organism as a result of genotype (ex. Tall) Chromosomes: non-dividing cell paternal maternal (c) jeridi Rubis locus alleles 2 Types of Genotype 1. Homozygous -pure-bred; having identical alleles for a particular trait 2. Heterozygous hybrid; having two different alleles for a particular trait Page 3 of 11 Genotype TT Tt tt Homozygous Heterozygous Homozygous Phenotype Tall Tall Short P= Parental (pure-breds / true-breeding) F₁ = First filial generation; offspring from P generation F2= Second filial generation of a genetic cross (F1 x F1) Monohybrid cross genetic cross involving a single pair of genes only (one trait) ex: height only Dihybrid cross - genetic cross involving two pairs of genes (two traits) ex: height and colour Mendel's Conclusion - He noticed that all offspring produced in the cross expressed only one of the two contrasting traits - He concluded: when true-breeding plants with contrasting traits are crossed, all offspring (100%) will express only one of the two traits Punnett Square useful tool to do genetic crosses - used to predict the genotypes and phenotypes of the offspring Probability - chance of getting a particular outcome over all possible outcomes - e.g. tossing a coin, rolling a dice, picking a card from a deck of cards (c) jeridi Rubis Page 4 of 11 Monohybrid Cross 1. Free earlobe is dominant over attached earlobes. Supposed a homozygous man for free earlobes reproduces with a woman with attached earlobes. 2. Legend: F - free earlobe f - attached earlobe Ot Legend: R-roller t - non-roller Ot R f r (c) jeridi Rubis A couple is both heterozygous tongue roller. f Rr KO R RR F Rr Ff r Rr ff rr a. Probability of them having a child with attached earlobe: 0% b. " " with free earlobe 100% 3 FF a. b. c. Rr Non-tongue rolling out of 8 children 1/4 of 82 children Phenotypic ratio 3 roller: 1 non-roller Probability of a tongue-rolling child 3/4 or 75% Mendel's Laws 1. Principle of Segregation - alleles of a gene separate when gametes are formed Page 5 of 11 2. Principle of Independent Assortment the segregation of alleles of one gene is independent of the segregation of alleles of another gene during gamete formation 1. In summer squash, white fruit colour is dominant over yellow fruit colour and disk-shaped fruit is dominant over sphere-shaped fruit. If a squash plant heterozygous for both traits is crossed with a plant true-breeding for yellow, sphere-shaped fruit, Dihybrid Cross mating involving two parents that differ in two genes (two independent traits) Legend: W - white w - yellow D-disk-shaped d - sphere-shaped Ot (c) jeridi Rubis WD Wd wD wd q 8 WwDd wwdd (WD, Wd, wD, wd) (wd) wd WwDd Wwdd wwDd wwdd a. Genotypic ratio 1 WwDd: 1 Wwdd: 1 wwDd: 1 wwdd b. Phenotypic ratio 1 white disk-shaped: 1 white sphere- shaped: 1 yellow disk-shaped: 1 yellow sphere-shaped c. Probability of yellow colour and disk- shaped offspring 1/4 or 25% Page 6 of 11 Non-Mendelian Genetics some traits don't follow the simple dominant/recessive rules that Mendel first applied to genetics - some alleles are neither dominant nor recessive - traits can be controlled by more than one gene Incomplete Dominance one allele is not completely dominant over another the heterozygous phenotype is a blending of the two homozygous phenotypes Ex. If you cross a RED (RR) snapdragon with a WHITE (WW), you get PINK (RW) snapdragons Other examples Hair Texture: Curly (CC), Straight (SS), Wavy (CS) Inter-eye Distance: Close-set (CC), Far apart (FF), Medium-set (CF) - Lip Protrusion: Protruding (PP). Non-protruding (NN), Slightly protruding (NP) Monohybrid Cross 1. In radish plants, the shape of radish may be long (LL), round (RR) or oval (LR). If oval is crossed with another oval, Legend: L- long R - round q LR LR a. What proportion of the offspring would also be oval? 1/2 or 50% (c) jeridi Rubis Ot L R L LL LR R LR RR Page 7 of 11 Dihybrid Cross 1. When a black Andalusian chicken is mated with a white Andalusian, the offspring is grey. Small size is dominant over the big-sized chickens. If a heterozygous small-grey coloured chicken is crossed with a big-black chicken. Legend: B - black W - white S - small s-big Ot SB SW (c) jeridi Rubis sB SW FO SSBW SSBB (SB, SW, sB, sW) (sB) sB SSBB SSBW SSBB ssBW a. Phenotypic ratio: 1 small-black: 1 small-grey: 1 big- black: 1 big-grey b. Probability of each offspring: small-black: 25% small-grey: 25% big-black: 25% big-grey: 25% Co-dominance two alleles are both expressed as a dominant phenotype coat colour in cows: CRCR: red CWCW: white CRCW: roan (white with red spots) Page 8 of 11 1. In horses, chestnut brown (CBCB) and white (CWCW) coat colours are co-dominant. Heterozygous horses have both colours, which is a golden tan colour. Such heterozygous horses are known as palominos (CBCW). Suppose two palominos were allowed to mate. Legend: CB - brown CW - white св си св (c) jeridi Rubis CBCW CW X CBCB CBCW CBCW CW CW 1. 3 CBCW a. Phenotypic ratio: b. c. 1 chestnut: 2 palominos: 1 white Genotypic ratio: 1 CBCB: 2 CBCW: 1 CWCW Multiple-Allele Series Co-dominance: more than two choices of alleles present for a trait -ABO blood type has three alleles: if both A and B are present, type is AB; neither is recessive Individuals can be type A, B, AB or O (recessive) In human blood groups: two alleles are expressed (multiple alleles) in heterozygous individuals example: blood type A: IAIA or IAi Probability: palominos: 1/2 or 50% white: 1/4 or 25% chestnut: 1/4 or 25% 2. type B: IBIB or Bi 3. type AB: 1A/B 4. type 0: ii Page 9 of 11 Practice Exercises 1. A woman homozygous for blood type B marries a man heterozygous for blood type A. Legend: JA - A IB-B |B |A|B i-0 2. A type B woman whose mother was O marries a type O man. q Legend: |A - A IB - B i-0 q |B|B Legend: C-agouti Cch- chinchilla Ch- himalayan c - albino (c) jeridi Rubis a. Phenotypic ratio: 1 AB: 1 B | Bi ii a. Phenotypic ratio: 1 B: 10 Coat Colour in Rabbits: C> Cch > Ch > C CCh |B ot FO JA a. Phenotypes: agouti, chinchilla, himalayan agouti > chinchilla > himalayan > albino (recessive) 3. Predict the phenotype for a rabbit with a Cch Ch and a CCh genotype. q X CchiCh C Cch CCch Ch CCh |Bi i |Bi i =: Ch Cch Ch Chch Page 10 of 11 Fraction of population Pelygenic Inheritance Polygenic Traits - poly = many genic = genes a vast number of possibilities - example: height, hair, skin and eye colour in humans the distribution of polygenic traits through the population follows a bell-shaped (normal) curve ... 000 AaBbCe You %4 000 ●00 ○ 000 000 000 (c) jeridi Rubis x 000 ... 000 AaBbce produced by skin cells. At least three genes conmol synthesis of melanin Each genc has two forma ne for high melanin production or dark skin (AJ dan allele for low melanin production or ght skin (Abc). Each dark skin alice (ABC) in the genotype adds a small but equal of pigment to the skin. There are seven different shades of skin color ranging from very dark (AABBOC) very light (l) Mividad peshare of melanin and are of intermedia The following Pet square shows the possible offering from a cross between two individuals of Gametes ABC Abe The Case of Desiree's Baby cont Part 11-Skin Color is a Polygenetic (Multiple Gene) Trait Polygenic traits. affe AaBbCe X ABCe Careers the of dark skin allen Alle AMC A ABC ARC 6 3 AABBC AABBC AABICC AAC Acc Aance ACC Alca 4 AABIC AABAABC AAB AC Alle Alle A AABBCC AAC AAMCC AAC AalbCC Acc AC Abbo AABC AABlee AAC AAbee Aalce Aabee Aabe Abbe 4 AaBCC Aalle AalbCC AalceaalsCC alice sallbCC alibe AaBBC AaBee AabCe Aalboe BBC Bec Caboc AalbCC Alice AabeCC AabCe aaBCC albe sabbCC acc Aabe Aalbec AubbCe Aabber Ce afbee The offspring of this cross exhibit seven shades of skin color based on the number of dark skin alleles in Page 11 of 11