1. 8th Grade Unit 2 Chapter 5, Section 2
Genetics Since Mendel
8th Grade
Unit 2
Chapter 5, Section 2

2. Most of your inherited traits involve more complex patterns of inheritance than Mendel discovered

3. Incomplete Dominance
Mendel’s experiments were repeated in the early 1900s
For some plants, such as peas, the results still proved true
Some varieties of plants produced different results
Purebred Japanese red four o’clock flowers crossed with purebred white four o’clocks should have produced heterozygous dominant red flowers in the offspring, but instead the offspring were pink!

4. In the Japanese four o’clock flower, the pink offspring were an intermediate phenotype
The apparent blending would become known as incomplete dominance

8. Mendel studied traits in peas that were controlled by just 2 alleles
Many traits are controlled by more than 2 alleles
Said to be controlled by multiple alleles
Produce more than 3 phenotypes

9. Human blood type is an example of multiple alleles that produces only 4 phenotypes
The alleles are A, B, and O
O type is recessive to both A and B, which are both dominant
Genotype/phenotype AB is codominant
Genotype for O is OO
Genotype for A is AA or AO
Genotype for B is BB or BO

10. Example of Multiple Alleles:
A man who is blood type AB marries a homozygous A woman. What are the offspring genotype and phenotype probabilities?
AB x AA F1 Genotype = 50% AA, 50% AB
F1 Phenotype = 50% A, 50% AB A A A AA AA AB AB B

11. Polygenic Inheritance
Eye color is an example of a trait produced by a combination of many genes
Polygenic inheritance
Height, eye color, skin tone (3-6 genes may control skin color)
Environment plays a role in the expression of a trait

12. Environmental influences can be internal or external
UV light and skin cancer
White fur in snowy habitats
Bright feathers on male birds (chemical/internal signaled by testosterone)

13. Human Genes & Mutations
Occasionally, errors occur in the DNA when it is copied during interphase of the cell cycle
These errors are called mutations
They can be harmful, helpful, or have no effect on the organism
Certain chemicals may produce mutations
As well as x-rays and radioactive substances
In addition to individual gene mutations, incorrect chromosome numbers may be inherited
Due to mistakes in meiosis (creation of egg/sperm)
Changes in total chromosome number in humans is usually fatal or the baby may die soon after birth
Trisomy 21 (3 copies of chromosome 21) results I Downs Syndrome, and they may live long lives if no health issues arise

16. Recessive Genetic Disorders
Many human genetic disorders are caused by recessive genes
Result of mutation within the gene (misreading of the mRNA)
Both parents must have the recessive allele
They are heterozygous for the trait and don’t show symptoms of the disorder
If the child inherits both recessive alleles they will display the disorder

19. Gender Determination
Fruit flies were studied to determine genetic inheritance
They only have four large chromosomes
Identified one pair as sex determining (XX = female, XY = male)

20. Sex-Linked Disorders
A sex-linked gene is an allele on a sex chromosome
Red-green color blindness is located on the X chromosome
Females have it only if both X chromosomes have the recessive allele
Males only have one X chromsome, so if that X has a recessive allele they have the trait

21. When reading pedigrees…
Dominant sex-linked disorders are rare and result when at least 1 dominant allele is inherited
Vitamin D-resistant rickets is X-linked dominant disorder
Kidneys can’t absorb sufficient phosphorus = soft bones and poor teeth
When reading pedigrees…
Males are squares, females are circles
Completely shaded is the trait is shown in that person
Half-colored shapes indicate a carrier of the trait, but it is not seen
Empty shapes do not have the trait and are not carriers