AQA A level Bio 7.1 Inheritance

Explain what is meant by the term phenotype (2)

Expression/appearance/characteristic due to genotype/alleles (1)

and the environment (1)

Tay-Sachs disease is a human inherited disorder. Sufferers of this disease often die

during childhood. The allele for Tay-Sachs disease t, is recessive to allele T, present in

unaffected individuals. The diagram shows the inheritance of Tay-Sachs in one family

Explain one piece of evidence from the diagram which proves that the allele for Tay-Sachs disease is recessive (2)

Unaffected 3 and 4 have affected offspring (1)

Both 3 and 4 are carriers (1)

Suggest two reasons why observed ratios are often not the same as expected ratios (2)

Fertilisation is random OR fusion of gametes is random (1)

Selective advantage (1)

Explain what is meant by codominant alleles (1)

Alleles that both contribute to phenotype in the heterozygous condition (1)

Human ABO blood groups are determined by the presence or absence of two antigens (A and B) on plasma membrane of the red blood cells. The plasma membrane of the red blood cells. The inheritance of these blood groups is controlled by three alleles.

I^A – determines the production of antigen A

I^B – determines the production of antigen B

I^o – determines the production of no antigen

Alleles I^A and I^B are codominant. Allele I^o is recessive to both. The pedigree shows the pattern of inheritance of these blood groups in a family over three generations.

How many antigen-determining alleles will be present in a white blood cell? Give reason for your answer (1)

2, inherited one from each parent(maternal and paternal) (1)

Human ABO blood groups are determined by the presence or absence of two antigens (A and B) on plasma membrane of the red blood cells. The plasma membrane of the red blood cells. The inheritance of these blood groups is controlled by three alleles.

I^A – determines the production of antigen A

I^B – determines the production of antigen B

I^o – determines the production of no antigen

Alleles I^A and I^B are codominant. Allele I^o is recessive to both. The pedigree shows the pattern of inheritance of these blood groups in a family over three generations.

If individuals 6 and 7 were to have another child, what is the probability that this child would be male and blood group A? explain your answer (3)

12.5 % (1)

parents I^B I^o and I^AI^o (1)

25% probability of blood group A and half will be male (1)

Explain what is meant by a dominant allele? (1)

Gene that is always expressed (1)