Biology 2B

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 Bb Bb b 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 BB 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

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 Bb B b BB Bb B b Bb bb

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 b b B Bb Bb b bb bb

7. Autosomal co-dominance SBSB Individuals with two of the 1st allele show the first trait Individuals with two of the 2nd allele show the second trait Individuals with one of each allele show a mixture of both traits SWSW SBSW

8. Autosomal co-dominant crosses SBSB SWSW 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 SBSW SBSW SB SW SB SBSB SBSW SW SBSW SWSW

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% SBSW SBSW SB SW SB SBSB SBSW SW SBSW 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 don’t carry an allele)

11. 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. Genetics problems 1

12. 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. Genetics problems 1 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 = tabby t = black Cross is Tt x tt Genotypes ½ Tt, ½ tt Phenotypes ½ tabby, ½ black t t H = short h = long Cross is Hh x hh Genotypes ½ Hh, ½ hh Phenotypes ½ short, ½ long h h T Tt tt H Hh hh t Tt h Hh hh tt 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. 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 A = astigmatism a= normal vision Cross is Aa x aa Genotypes ½ Aa, ½ aa Phenotypes ½ astigmatism, ½ normal t t a a T Tt tt A Aa aa T Tt tt a Aa aa

13. 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 Genetics problems 2 Red flower colour is co-dominant with white flower colour in snap dragons, with the heterozygote showing pink flowers

14. 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 Genetics problems 2 Red flower colour is co-dominant with white flower colour in snap dragons, with the heterozygote showing pink flowers FRFR = red, FWFW = white, FR FW = pink FRFR x FW FR FR FW FWFW x FRFR  allFWFRpink FR FRFR FRFW ½ FRFR = red, ½ FRFW = pink FR FRFR FRFW 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 FWFW x FW FR FR FW FR FW FW FRFW FWFW FRFW x FR FW FR FRFR FRFW FWFW FW FRFW FRFW FWFW FW ½ FRFR = red, ½ FRFW = pink 1 FRFR = red : 2 FR FW = pink : 1 FWFW = white

15. 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? Genetics problems 3 Haemophilia is a disease carried as a sex-linked recessive trait.

16. 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? Genetics problems 3 Haemophilia is a disease carried as a sex-linked recessive trait. XHXH = normal female, XHXh = carrier female, XhXh = haemophiliac female XHY = normal male, XhY = haemophiliac male XHXH x XhY XHXh x XHY XH XH XH Xh All females carriers XHXh all males normal XHY Xh XHXh XHXh XH XHXH XHXh Y XHY XHY Y XHY XhY 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? XH Xh XH Xh XHXh x XhY XHXh x XhY Xh XHXh XhXh Xh XHXh XhXh Probability is 1/4 Y Y XHY XhY XHY XhY ½ females are XHXh = carrier, ½ females are XhXh = haemophiliac ½ males are XHY = normal, ½ males are XhY = haemophiliac