IBDP>CORE TOPICS>TOPIC 4 GENETICS>4.3 THEORETICAL GENETICS
Describe the inheritance of ABO blood groups. 9marks
One gene determines (ABO) blood groups.
Genes have different/alternative forms called alleles; there are three alleles (IA, IB and i) of the gene for (ABO) blood groups;
ABO blood groups are an example of multiple alleles. (in this instance three alleles can result in four phenotypes);
Each individual has two alleles of the gene but only one is passed to offspring;
Alleles that are codominant both affect the phenotype in a heterozygote; (alleles) IA and IB are codominant; (alleles) IA and IB are dominant over i / i is recessive to IA and IB;
IA IA and IA i both give blood group A;
IBIB and IBi both give blood group B;
IAIB gives blood group AB;
ii/homozygous i gives blood group O;
Example of a cross involving ABO blood groups can be given
Genes have different/alternative forms called alleles; there are three alleles (IA, IB and i) of the gene for (ABO) blood groups;
ABO blood groups are an example of multiple alleles. (in this instance three alleles can result in four phenotypes);
Each individual has two alleles of the gene but only one is passed to offspring;
Alleles that are codominant both affect the phenotype in a heterozygote; (alleles) IA and IB are codominant; (alleles) IA and IB are dominant over i / i is recessive to IA and IB;
IA IA and IA i both give blood group A;
IBIB and IBi both give blood group B;
IAIB gives blood group AB;
ii/homozygous i gives blood group O;
Example of a cross involving ABO blood groups can be given
Describe the role of sex chromosomes in the control of gender and inheritance of hemophilia. 7marks
Sex chromosomes are X and Y; one sex chromosome is inherited from each parent;
XX results in female; XY results in male; sex is determined by sperm/father.
sex-linked genes are those located on the sex chromosomes / usually refers to genes on X chromosome; recessive sex-linked traits appear more frequently in males since they only have one X chromosome;
Hemophilia is an example of a gene located on the X chromosome/sex-linked; female carriers are heterozygous / XHXh; males with hemophilia are XhY / normal males are XHY;
sons (of carrier females) have 50 % probability of showing the trait (even if father is normal); daughters (XhXh) of hemophiliac father and carrier mother can be affected / daughters who receive an affected X from each parent will have haemophilia;
The points above can be gained by annotated Punnett squares. Candidates may introduce a lettering system for haemophilia genotypes which does not include H and h. Accept other letters for superscripts, but same alphabetical letter should be used throughout, dominant form should appear as upper case letter and recessive as lower case letter.
XX results in female; XY results in male; sex is determined by sperm/father.
sex-linked genes are those located on the sex chromosomes / usually refers to genes on X chromosome; recessive sex-linked traits appear more frequently in males since they only have one X chromosome;
Hemophilia is an example of a gene located on the X chromosome/sex-linked; female carriers are heterozygous / XHXh; males with hemophilia are XhY / normal males are XHY;
sons (of carrier females) have 50 % probability of showing the trait (even if father is normal); daughters (XhXh) of hemophiliac father and carrier mother can be affected / daughters who receive an affected X from each parent will have haemophilia;
The points above can be gained by annotated Punnett squares. Candidates may introduce a lettering system for haemophilia genotypes which does not include H and h. Accept other letters for superscripts, but same alphabetical letter should be used throughout, dominant form should appear as upper case letter and recessive as lower case letter.