1. Chapter 14: Human Inheritance
Section 14-1: Human Chromosomes

2. Studying Humans?
Human heredity is difficult to study because they produce few offspring, have a long life span, and cannot be grown in a lab

3. Karyotypes
Need to study the human genome – the full set of genetic information an organism carries in its DNA
Genomic study starts with chromosomes
To see chromosomes clearly, scientists, take a picture of the cell going through mitosis, cut the chromosomes out the picture and make a karyotype
A karyotype is a diagram that shows the complete set of diploid chromosomes grouped together in pairs and arranged in order of decreasing size
Normal human karyotype contains 46 chromosomes (23 pairs)

5. Sex Chromosomes
Two of the 46 (one pair) are sex chromosomes (determines an individual’s sex)
Females have two X chromosomes
Males have one X and one Y
Roughly a 1:1 ratio of females to males

6. Sex Chromosomes
The X chromosome is large and carries more than 1200 genes, most of which are necessary for both sexes
The Y chromosome is much smaller, only carrying 140 genes, controlling male sex determination and sperm development

7. Autosomal Chromosomes
The other 44 human chromosomes are known as autosomal chromosomes, or autosomes
Biologists write 46,XX for females and 46,XY for males

8. Transmission of Human Traits
Many human traits are Mendelian (simple dominance)
Ex. - gene known as MC1R helps determine skin and hair color
Some of MC1R’s recessive alleles produce red hair, while dominant alleles for the MC1R gene help produce darker hair colors
Ex. – Rhesus factor, or Rh blood group
The allele for Rh factor comes in two forms: Rh+ and Rh-.
Rh+ is dominant, so an individual with both alleles (Rh+/Rh-) has Rh positive blood.
Rh negative blood is found in individuals with two recessive alleles (Rh+/Rh-)

9. Codominant and Multiple Alleles
ABO blood group, determined by 3 alleles IA, IB, and i
Alleles IA and IB are codominant - produce molecules known as antigens on the surface of red blood cells.
Individuals with alleles IA and IB produce both A and B antigens, making them blood type AB

10. Codominant and Multiple Alleles
The i allele is recessive – no antigens
If a patient has AB-negative blood, it means the individual has IA and IB alleles from the ABO gene and two Rh- alleles from the Rh gene

11. Sex-Linked Inheritance
Genes located on X or Y chromosomes are called sex-linked genes
Genes on Y chromosome are found only in males, passed directly from father to son
Genes on X chromosome are found in both sexes
Males only get one X

12. Sex-Linked Inheritance
There are 3 genes responsible for color vision, all located on the X chromosome
In males, any of these genes being recessive causes colorblindness, the most common form affecting 1 in 12 males
This condition only occurs in 1 in every 200 females because she must have two recessive alleles
The recessive phenotype of a sex-linked genetic disorder tends to be much more common among males than among females (hemophilia, DMD)
Males cannot be heterozygous for genes carried on the X-chromosome!!

13. X-Chromosome Inactivation
Just one X chromosome is enough for cells in males, but females have 2
In female cells, most of the genes in one of the X chromosomes are randomly switched off, forming a dense region in the nucleus known as a Barr body
Barr bodies are not found in males

14. X-Chromosome Inactivation
Happens in other mammals as well – calico cats
Gene that controls the color of coat spots is located on the X chromosome
In a females, one X chromosome may have an allele for orange spots and the other X chromosome may have an allele for black spots
Depending on which X chromosome is turned off in which cell, the spots can be either color
As a result, the cat’s fur has a mixture of orange and black spots

15. X-Chromosome Inactivation
Male cats only have one X chromosome, so they can only have spots of one color
If a cat’s fur has 3 colors, it must be a female!

16. Human Pedigrees
A pedigree is a diagram that shows the inheritance of a single trait through multiple generations of a family
It shows the relationship betweens parents, siblings, and offspring
Based on family history and medical records