1. Inheritance
Human Biology

2. Keywords Chromosomes Homologous chromosomes Locus (loci pl.) Alleles
Dominant
Recessive
Codominant
Multiple alleles
Heterozygous
Homozygous
Sex-linked chromosomes
Punnet squares

3. Chromosomes
The genes for a particular trait occur at a particular point in the chromosome called a locus
There are 2 alternative forms of a gene called alleles

4. Chromosomes
Map of chromosome 7
ncbi.nlm.nih.gov

5. Alleles & dominance
Alleles can be:
Dominant
Recessive
Co-dominant

6. Alleles & dominance Dominant/recessive genes
The dominant allele will mask the trait of the recessive allele
If the genotype is heterozygous for both dominant and recessive alleles, the phenotype will only show the dominant trait
Co-dominant genes
Co-dominant alleles are neither recessive nor dominant
If the genotype is heterozygous for co-dominant alleles, each allele contributes equally to the phenotype

7. Tay-Sachs disease: recessive allele
Tay-Sachs is lipid metabolism disorder caused by a missing enzyme.
This results in the accumulation of a fatty substance in the nervous system
Death usually occurs at age 4 or 5

9. Sickle-cell anaemia: codominant allele
Being heterozygous for the sickle-cell gene means increased immunity against malaria
A person who is homozygous for the sickle-cell gene will have the disease, which is usually fatal
A person who is heterozygous for the sickle-cell gene will show slight sickling of the red blood cells, but their health will not be affected

10. Multiple alleles
Often there are more than 2 alleles in the general population for a particular characteristic
These are multiple alleles and the locus of that gene is refered to as multi-allelic
Each individual will only have 2 alleles, one inherited from the father and one from the mother

11. Multiple alleles
The ABO blood group system is based on the fact that an individual can possess any two of three alternative alleles: Ia, Ib, i
These are found on chromosome 9
They are responsible for 2 different antigens (proteins) that are found on the membranes of red blood cells
Ia allele produces antigen a, allele Ib produces antigen b and i produces no antigens

12. Multiple alleles
Alleles Ia and Ib are codominant that means that neither allele is dominant to the other and both characteristics take place.
i allele is recessive to both Ia and Ib
Situations where multiple alleles are involved are called polymorphic. Polymorphism is where many different physical forms can result from the various alleles at a single locus
Polymorphic genes are genes that may exist in 3 or more allelic forms
Multiple alleles result from different mutations of the same gene

13. ABO blood group system

14. ABO blood group system

15. Sex determination
The 23rd chromosome is either an X chromosome or a Y chromosome
Males XY
Carry gametes with either X or Y chromosomes
Females XX
Carry gametes with only X chromosomes

16. Sex-linked characteristics
The Y chromosome is smaller than the X chromosome
The X chromosome has the full complement of genes
Many of the genes on the X chromosome do not have matching alleles on the Y chromosome
Therefore, if there is a mutation in a gene on the X chromosome, then it will be expressed in the phenotype
These different patterns are called sex-linked characteristics
Examples
Red/green colour blindness
Haemophilia

17. Colour blindness: sex linked characteristic

18. Colour blindness: sex linked characteristic

19. Haemophilia: sex linked characteristic

20. Punnett squares
Punnett squares show the inheritance patterns of offspring for a particular gene
Punnett squares can be used to show monogenic inheritance and simple sex-linked inheritance traits
Punnett squares cannot show polygenic inheritance patterns (these involve multiple genes)
Alleles are represented by letters
A capital letter represents a dominant trait
A small case letter represents a recessive trait
One allele comes from the father, the other from the mother

21. Punnett squares Dominance & co-dominance

22. Punnett squares: colour blindness
Who passes on colour blindness?

23. Punnett squares: colour blindness

24. Punnet squares
Bozeman science 12 minutes