1. Genetics Since Mendel
Chapter 5, Section 2

2. Genetics Since Mendel What have we found out since Mendel?
Since Mendel’s experiments, scientists have found that genetic inheritance can be much more complicated than the simple, dominant-recessive inheritance that Mendel saw in pea plants.

3. Incomplete Dominance Incomplete Dominance
Occurs when offspring of two homozygous parents show an intermediate phenotype.
The heterozygous form is in between the dominant and recessive homozygous forms.

4. Incomplete Dominance Incomplete Dominance Example: Snapdragons:
Crossing a purebred white snapdragon with a purebred red snapdragon produces a pink snapdragon.

5. Multiple Alleles Multiple Alleles
Trait is controlled by more than two alleles.
Produce more than 3 phenotypes for the trait.

6. Multiple Alleles Example: Multiple Alleles Blood Type:
Alleles: IA IB and i
Phenotypes: A, B, AB, O
Type O is recessive to types A and B.
Types A and B are both expressed in the phenotype when paired together.

7. Polygenic Inheritance
Occurs when a group of gene pairs act together to produce a trait.
Traits are controlled by alleles from more than one gene.
A wide variety of phenotypes are produced.
Examples:
Hair, eye, and skin color

8. Impact of the Environment
Environmental influence on genetics:
Environmental influences can be internal or external.
Examples:
A boy who has genetics to be six feet tall is malnourished as a child and is only five foot eight inches tall.
Male birds exhibit brighter feathers than female birds.

9. Human Genes and Mutations
What is a mutation again?
Error in the DNA sequence.
Can be harmful, helpful, or null.
Can be caused by mistakes in replication or by external agents called mutagens.

10. Human Genes and Mutations
Chromosome Disorders
An organism receives too many or too few chromosomes as a result of mistakes in meiosis.
Usually fatal
Example:
Down’s syndrome
3 copies of chromosome 21.

11. Recessive Genetic Disorders
Disorder only appears if an individual has both recessive alleles.
Parents are carriers (heterozygous) and do not show symptoms of the disorder.

12. Recessive Genetic Disorders
Example:
Cystic fibrosis
Production of thick instead of thin mucous in the lungs and intestinal tract.
Most common genetic disorder leading to death in Caucasian Americans.

13. Determination of Gender
Boy or Girl?
Sperm cells have either an X or a Y chromosome.
Egg cells have only an X chromosome.
Males – XY
Females - XX

14. Sex-Linked Disorders Sex-linked Disorders
Alleles controlling the genetic disorder are linked to either the X or the Y chromosome.

15. Sex-Linked Disorders Sex-linked Disorders Example: Color blindness
Recessive allele linked to the X-chromosome.
All males with the recessive allele are colorblind.
Females must have two recessive alleles to be colorblind.

16. Pedigrees Pedigrees Family tree for a certain trait.
Used to determine inheritance patterns and probabilities and in breeding plants and animals.

17. Pedigrees How to read a pedigree: Males – squares Females – circles
Have the trait – colored
Does not have the trait – empty
Is a carrier (heterozygous) – half-colored