1. Human Genetics
Chapter 12

2. Pedigrees A pedigree is a graphic representation of human inheritance.
Squares = males
Circles = females
Shaded circles/squares =
individuals showing the
trait being studied.

3. Recessive Heredity
Most genetic disorders are caused by recessive alleles (must be inherited from both parents)
Traits with recessive alleles usually skip a generation.
Examples – cystic fibrosis, Tay-Sachs disease, phenylketonuria

4. Simple Dominant Heredity
Only need to be inherited from one parent.
Examples – tongue rolling, attached earlobes, hitchhikers thumb, Huntington’s disease

5. Incomplete Dominance
The phenotype of the heterozygous is intermediate.
Ex. Red flower (RR) x white flower (rr) = pink flower (Rr)
Ex. Curly Hair (HH) x straight hair (hh) = wavy hair (Hh)

6. Codominance
Causes the phenotype of both homozygous parents to be produced in heterozygous offspring.
Example:
black feather chicken (BB) + white feather
chicken (bb) = black and white feather chicken (Bb)

7. Other Genetic Patterns
Multiple Alleles – traits controlled by more than two alleles.
Sex determination:
autosomes (44) – all the other chromosomes
sex chromosomes (2) – determine sex
XX = female
XY = male

8. Sex-linked traits – traits controlled by genes located on the sex chromosome.
* usually occur more in males
Polygenic inheritance – traits controlled by two or more genes (Many alleles)
ex. eye color and skin color

9. Codominance In Humans Sickle-cell Anemia:
- most common in black Americans (1 in 12)
- the presence of abnormal shaped red blood
cells (sickle shaped or half moon)
- slow blood flow, block small vesssels, and
result in tissue damage
- Individuals who are heterozygous, produce
both normal and sickle shaped blood cells.
(codominance)

11. Multiple Alleles in Humans
Blood Type:
- Gene I has three alleles: IA, IB, i
- these three alleles produce four blood types
A, B, AB, and O
- blood typing is necessary before a person
can get a blood transfusion.
- blood typing can also help determine
parents

14. Sex-Linked Traits in Humans
Traits that are determined by genes that are carried on the sex chromosomes.
Most are located on the X-chromosome.
Males have these traits more than females
Ex. Colorblindness, hemophilia

15. Polygenic Inheritance in Humans
Eye Color
Skin color

16. Changes in Chromosome Numbers
Humans normally have 44 autosomes (22 pairs)
- an extra chromosome is called trisomy
- Trisomy 21 (extra 21st chromosome) = Down
Syndrome
Changes in sex chromosomes:
- If there is a Y chromosome, it is male.
- If there is an X chromosome, it is female
- Extra chromosomes usually mean they are
sterile, but can lead normal lives