1. Human Genetic Traits
Genetics-Unit 3, Part A

2. I.) Basic Information
Humans have 46 chromosomes, organized into 23 pairs
They are organized by sizes, then numbered from 1-22, #1 is the largest, #22 the smallest – called a karyotype
in females – XX in males – XY
The first 22 pair are called autosomes

3. II.) Types of Inheritance w/examples
Four ways traits are inherited:
1.) autosomal dominant
2.) autosomal recessive
3.) sex-linked dominant
4.) sex-linked recessive
In order to study these traits we must have a way to follow them over many generations. This is called a pedigree.

4. Symbols used for pedigrees:
Fraternal twins
Mating of family/cousins
male
female
death
mating
Identical twins
stillborn
siblings
affected

5. 1) Autosomal Dominant Many common traits are inherited by this method
A few diseases are also inherited this way, such as “wooly” hair and Huntington’s disease
What a pedigree would look like: I II
III

6. What the pedigree shows (general characteristics):
Every affected person must have an affected parent
The trait almost never skips generations
An affected heterozygote will usually transmit the trait to half his offspring

7. 2) Autosomal Recessive
Although some “normal” traits are recessive, most diseases are caused by recessive genes
Examples include PKU, sickle-cell anemia, and albinism
Pedigree: I II
III

8. Characteristics
Most affected individuals have two normal parents (who are heterozygotes)
If the trait is rare, an affected individual will probably marry a normal individual, producing heterozygous normal offspring
Matings between heterozygotes usually show a 3:1 ratio of normal to affected offspring
When both parents are affected, all their progeny will also be affected

9. Sex-linked dominant
These are rare, dominant traits carried on the X-chromosome
Diseases inherited in this way include a form of schizophrenia and faulty tooth enamel
Pedigree: I II
III

10. Cont……
Males can’t pass the disease to their sons, do pass it to all their daughters
Females pass the disease equally to both children, ½ of them will inherit the trait
Females seem to have a milder case of the trait
Females (with 2 XX’s) typically have the trait more than males

11. Sex-linked recessive
Most common type of inheritance for genetic diseases
A plethora of traits inherited in this fashion
Pedigree:

12. Cont…… Females pass disease to their sons, may make daughters carriers
Males don’t pass disease to their sons, can make daughters carriers

13. Gene Expression and the Environment
Although the genetics of an organism is key to its survival, those genes are very dependent on the environment both to how they are expressed and when
Gene expression and environmental influence will lead to the development of the organism. Development can be characterized as a 4-step process:

14. Process Replication, transcription, translation of genetic material
Growth
Differentiation into various cell types
Aggregation of cells into tissues and organs
Really, no gene is wholly responsible for development, the environment has some influence on the expressed phenotype

15. Penetrance and Expressivity
Penetrance is defined as the frequency with which a dominant or homozygous recessive gene shows itself in individuals of a population
Penetrance is dependent on both genotype and environment
In some cases, not all organisms of a given genotype show the resulting phenotype, due to the environment’s influence

16. Cont…. Penetrance may be complete or incomplete
complete –100 % show the trait
incomplete- < 100% show the trait
Ex. Corn Plants
100 % of population is Tt, yet 5 plants are short. Why?
Each plants’ environment may affect its growth

17. Human examples: (That’s you and me!)
Brachydactly (autosomal dom. trait – shorter than normal index fingers & toes) Shows 50-80% penetrance
Neurofibromatosis (body tumors) – again 50-80% penetrance – “Elephant man” – over 80% penetrance

18. Express yourself in ….Expressivity
Defined as the degree to which a gene is expressed, usually as “mild” or “moderate”, or “severe”, again a range of symptoms or phenotypes may be observed.

19. Let’s learn more on the environment
Effects of Internal & External Environment
A. Internal Environment
Age & sex are the biggest factors which contribute to gene expression.
As organisms age, genes may be “turned on or off”, causing changes in the organism.
Example include age influenced traits like Huntington’s & pattern baldness.
Two types of autosomal traits are under the control of the sex of the individual. These are known as sex-limited and sex-influenced traits.

20. Oh yes, there is more…
Sex-limited traits are those where both male and female have the gene, but only 1 sex shows the trait. (Ex. Feathers in chickens, milk prod. in cows, facial hair in humans.)
Sex influenced traits are where both sexes show the trait, but one shows it more. Horn size in animals, pattern baldness.

21. That’s it for internal…but next is the ex.
B. External Environment
Main factors are temp., nutrition, light, chemicals, and infectious agents.
Examples:
enzymes can be affected by small changes in temperature. Himalayan rabbits’ fur color can be affected by slight changes in temperature.
at > 30°, the rabbits will be all white
at 25°, rabbits’ extremities are brown
at < 25°, extremities and skin patches are brown
PKU, diet can influence symptoms
if phenyalanine is left out of the diet, symptoms are less severe