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Biology 2B Genetics. Genetic problems A monohybrid cross is a cross of individuals looking at a characteristic inherited at one gene locus A test cross.

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Presentation on theme: "Biology 2B Genetics. Genetic problems A monohybrid cross is a cross of individuals looking at a characteristic inherited at one gene locus A test cross."— Presentation transcript:

1 Biology 2B Genetics

2 Genetic problems A monohybrid cross is a cross of individuals looking at a characteristic inherited at one gene locus A test cross is crossing an individual back to a homozygous recessive individual in order to determine whether it is a carrier A Punnett square is a tool used in genetics Genotype refers to the alleles present in an individual Phenotype refers to the characteristic shown by the individual b B b b Bb bb Bb bb

3 Autosomal inheritance Both males and females have 2 alleles for the characteristic Homozygous individuals have 2 alleles the same and produce gametes with only 1 type of allele Heterozygous individuals have 2 different alleles and produce two types of gametes with each allele At fertilisation gametes combine so the new individual has 2 of each allele – one from each parent We can show the probabilities of allele combinations from different crosses by using a Punnett square

4 Autosomal dominant/recessive Individuals with two dominant alleles show the dominant phenotype Individuals with two recessive alleles show the recessive phenotype Individuals with one of each allele show the dominant phenotype BB bb Bb

5 Autosomal dominant recessive crosses Crossing a homozygous dominant individual with a homozygous recessive individual leads to offspring who are all heterozygous and show the dominant trait Crossing two heterozygous individuals leads to 1 homozygous dominant individual, showing the dominant trait : 2 heterozygous individuals, showing the dominant trait :1 homozygous recessive individual, showing the recessive trait BB bb Bb BBBb bb Bb B b

6 Example – dominant recessive problem A heterozygous black male mouse mates with a homozygous brown female mouse. Black fur is dominant over brown fur. What is the probability of having: a) a homozygous black offspring? 0% b) a heterozygous black offspring? 50% c) a homozygous brown offspring? 50% Bb bb bb BBb bbb

7 Autosomal co-dominance Individuals with two of the 1 st allele show the first trait Individuals with two of the 2 nd allele show the second trait Individuals with one of each allele show a mixture of both traits SBSBSBSB SWSWSWSW SBSWSBSW

8 Autosomal co-dominant crosses Crossing an individual homozygous for one allele with an individual homozygous with the second allele leads to offspring showing a mixture of the two traits Crossing two heterozygous individuals leads to 1 homozygous individual showing the first trait : 2 heterozygous individuals showing the mixed trait :1 homozygous individual showing the second trait SBSBSBSB SWSWSWSW SBSWSBSW SBSWSBSW SBSB SWSW SBSB SBSBSBSB SBSWSBSW SWSW SBSWSBSW SWSWSWSW

9 Example – co-dominance problem Two heterozygous grey sheep are mated. Black wool is co- dominant to white wool. What is the probability of having: a) a black offspring? 25% b) a grey offspring? 50% c) a white offspring? 25% SBSWSBSW SBSWSBSW SBSBSBSB SBSWSBSW SBSWSBSW SWSWSWSW SBSB SWSW SBSB SWSW

10 Sex linked inheritance Males and females have different chromosomes Males can only show 2 phenotypes (ie males can not be carriers) Females can show 3 phenotypes (if codominant) or 2 phenotypes (if dominant recessive, with a carrier) You need to show alleles on the X chromosome (Y chromosomes dont carry an allele)

11 Genetics problems 1 Tabby is dominant to black coat colour in cats. Predict the genotype and phenotype ratios expected from a cross between a hybrid tabby and a black cat. Tongue rolling is dominant to non- tongue rolling in humans. Predict the genotype and phenotype ratios expected from a cross between a heterozygous tongue roller and a non tongue roller. Short hair is dominant to long hair in cats. Predict the genotype and phenotype ratios expected from a cross between a hybrid short-haired cat and a long hair cat. Astigmatism is dominant to normal vision in humans. Predict the genotype and phenotype ratios expected from a cross between a person heterozygous for astigmatism and a normally sighted person.

12 Genetics problems 1 Tabby is dominant to black coat colour in cats. Predict the genotype and phenotype ratios expected from a cross between a hybrid tabby and a black cat. Astigmatism is dominant to normal vision in humans. Predict the genotype and phenotype ratios expected from a cross between a person heterozygous for astigmatism and a normally sighted person. T = tabby t = black Cross is Tt x tt Genotypes ½ Tt, ½ tt Phenotypes ½ tabby, ½ black Tongue rolling is dominant to non-tongue rolling in humans. Predict the genotype and phenotype ratios expected from a cross between a heterozygous tongue roller and a non tongue roller. Short hair is dominant to long hair in cats. Predict the genotype and phenotype ratios expected from a cross between a hybrid short-haired cat and a long hair cat. t t TTt tt tTt tt hh HHhhh hHhhh tt TTttt TTttt aa AAaaa aAaaa T = tongue roller t = non tongue roller Cross is Tt x tt Genotypes ½ Tt, ½ tt Phenotypes ½ tongue roller, ½ non tongue roller H = short h = long Cross is Hh x hh Genotypes ½ Hh, ½ hh Phenotypes ½ short, ½ long A = astigmatism a= normal vision Cross is Aa x aa Genotypes ½ Aa, ½ aa Phenotypes ½ astigmatism, ½ normal

13 Genetics problems 2 Predict the genotypes and phenotypes expected from crossing a white flowered plant and a red flowered plant Predict the genotypes and phenotypes expected from crossing a pink flowered plant and a white flowered plant Predict the genotypes and phenotypes expected from crossing a pink flowered plant and a red flowered plant Predict the genotypes and phenotypes expected from crossing 2 pink flowered plants Red flower colour is co-dominant with white flower colour in snap dragons, with the heterozygote showing pink flowers

14 Genetics problems 2 Predict the genotypes and phenotypes expected from crossing a white flowered plant and a red flowered plant Predict the genotypes and phenotypes expected from crossing a pink flowered plant and a red flowered plant Red flower colour is co-dominant with white flower colour in snap dragons, with the heterozygote showing pink flowers Predict the genotypes and phenotypes expected from crossing a pink flowered plant and a white flowered plant Predict the genotypes and phenotypes expected from crossing 2 pink flowered plants F R F R = red, F W F W = white, F R F W = pink F W F W x F R F R all F W F R pink F W F W x F W F R F R F R x F W F R F R F W x F R F W FRFWFRFW FRFR FWFW FRFRFRFR FRFR FRFR FRFRFRFR FRFWFRFW FRFR FWFW FWFWFWFW FRFWFRFW FWFW FWFW FRFWFRFW FWFWFWFW FRFR FWFW FRFRFRFR FRFWFRFW FRFR FWFWFWFW FWFW FRFWFRFW ½ F R F R = red, ½ F R F W = pink 1 F R F R = red : 2 F R F W = pink : 1 F W F W = white

15 Genetics problems 3 What genotypes and phenotypes are expected if a haemophiliac male marries a homozygous normal female? What genotypes and phenotypes are expected if a haemophiliac male marries a heterozygous female? What genotypes and phenotypes are expected if a normal male marries a heterozygous female? What is the probability of having a son with haemophilia, if a haemophiliac male marries a heterozygous female? Haemophilia is a disease carried as a sex-linked recessive trait.

16 Genetics problems 3 What genotypes and phenotypes are expected if a haemophiliac male marries a homozygous normal female? What genotypes and phenotypes are expected if a normal male marries a heterozygous female? Haemophilia is a disease carried as a sex-linked recessive trait. What genotypes and phenotypes are expected if a haemophiliac male marries a heterozygous female? What is the probability of having a son with haemophilia, if a haemophiliac male marries a heterozygous female? X H X H = normal female, X H X h = carrier female, X h X h = haemophiliac female X H Y = normal male, X h Y = haemophiliac male X H X H x X h Y X H X h x X H Y X H X h x X h Y Y XHXH XhXh XHXH XHXH XHXH XHXHXHXH XHXhXHXh XHYXHY XHXH XhXh XHXhXHXh XHXhXHXh XHXH XhXh XHXhXHXh XhXh XhXhXhXh XhXh XhXh XHXhXHXh XhXhXhXh XHYXHYXHYXHYYXhYXhY Y XHYXHYXhYXhY YXHYXHYXhYXhY All females carriers X H X h all males normal X H Y ½ females are X H X h = carrier, ½ females are X h X h = haemophiliac ½ males are X H Y = normal, ½ males are X h Y = haemophiliac Probability is 1/4


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