Chapter 12

Genetics: Study of heredity.

Heredity: The passing of traits from parents to offspring.

I. Mendel: Mendel: The Father of The Father of GeneticsGenetics

Has many traits that have two clearly different forms; easy to tell apart.

Mating can easily be controlled b/c the male and female reprod. parts are enclosed w/in the same flower.

Mendel used the garden pea :

Small, grows easily, matures quickly, and produces lots of offspring.

Self-pollinates (easy breeding)

Useful Features of Peas

Seeds inherit all traits from the plant that bore them!

Mendel’s First ExperimentsIn your book, p. 269!

Please read about Mendel’s experiment and simplify the steps by explaining them in

your own words.

(Pair-Share)

Mendel studied cross-pollination, which produces seeds that are the offspring of two different plants.* Mendel began with purebred plants (the result of self-pollination).

He studied the following traits: plant size, pod shape, seed color, seed coat, flower color.Hybrids – org. produced by crossing parents with different traits. (ex. Purple flower X white flower)

True-Breeding = All offspring displays same form of a trait. P generation = parents.F1 generation = offspring of the p generation.F2 generation = offspring of the F1 generation.

In familiar terms please…

P generation (grandpa)

F1 generation (dad)

F2 generation (baby)

All of Mendel’s F1 plants showed only one form of the trait. (i.e. all offspring had purple flowers)All of the different trait had disappeared!!

When he let the F1 plants self-pollinate, the missing trait reappeared in the F2 generation!

Mendel found that for all seven traits that he studied, a 3:1 ratio was observed in the F2 generations.

Summary

1.For each inherited trait, an individual has two copies of the gene—one from each parent.

2.There are alternative versions of each gene, called ALLELES.

Mendel’s Theory

3. When 2 different alleles occur together, one may be completely expressed, while the other “disappears”. do NOT blend!Dominant: The expressed form of a trait.Recessive: Is not expressed if a dominant allele is present.

Mendel’s Laws

Important Terms

Using the Frayer Model (reading strategy) explain the relationship between the following genetics terms, beginning on p. 273:

• Genotype vs. Phenotype •Homozygous vs. Heterozygous•Dominant vs. Recessive

1. Law of Segregation: When an organism produced gametes, each pair of alleles is separated and each gamete has an equal chance or receiving either one of the alleles.

Mendel’s Laws

2. Law of Independent Assortment: during gamete formation, alleles for each trait segregate independently.

• inheriting one trait doesn’t affect your inheritance of another trait

Mendel’s LawsMendel’s Laws

Types of Types of CrossesCrosses

MonohybridMonohybrid = involves one trait = involves one trait (Ex. seed color)(Ex. seed color)

DihybridDihybrid = involves two traits = involves two traits (Ex. Seed colod and seed (Ex. Seed colod and seed

shape)shape)

• Model that predicts the likely outcomes of a genetic cross.

• Shows all of the genotypes that could result!

Punnett Squares

A pedigree is a family history that shows how a trait is inherited over several generations.Circles = femalesSquares = malesShaded circle or square = trait expressed.Circle or square not shaded = trait not expressed.

Pedigrees

Autosomal traits =Located on chromosomes other than X and Y. Will appear in both sexes equally.A sex-linked trait is a trait whose allele is located on the X chromosome. Most sex-linked traits are recessive.

Why would a sex-linked disorder show up more in males than in females?Working sex-linked Punnett Squares:

A. Blood GroupsA. Blood GroupsABO Groups

Red Blood Cells (RBC) can carry two antigens, A & B

Presence & absence of A & B antigens produces 4 possible blood types: A, B, AB, O

B. Rh Groups+ if Rh antigen is present on RBC, - if not.

Phenotype GenotypeA IAIA, IA i B IBIB, IB i AB IAIB

O i i

Traits NOT controlled by simple dominant-recessive alleles

Ex. Height, weight, hair color, skin color in humans

Polygenic traits: Traits controlled by multiple genes.

Incomplete Dominance = Offspring displays a trait intermediate between the two parents.

Ex. Red + white = pink in snapdragons.

Ex. Straight + Curly = wavy hair in Caucasians.

Codominance = Two dominant alleles are expressed at the same time.

Ex. Red hair + white hair = roan hair in horses & cattle

Multiple Alleles = Genes with 3 or more alleles.

Ex. ABO blood group

Traits Influenced by the Traits Influenced by the EnvironmentEnvironmentAcidity of soil determines

flower color in hydrangeaTemperature affects coat

color of arctic fox.Fur on the ears, nose, paws,

and tail is darker in Siamese cats because body temperature is cooler.

Traits Caused by MutationTraits Caused by Mutation

1. Sickle Cell Anemia = A recessive disorder caused by a mutated allele that produces hemoglobin.

RBC take on a disk-like shape when oxygen levels are low.

Results in poor blood circulation.

1:500 African Americans

Protects affected people from malaria.

2. Hemophilia

RecessiveSex-linkedImpairs blood’s ability to clot.

1:10,000 white males

3. Huntington’s Disease Caused by a dominant allele on an

autosome. Symptoms begin to show in 30’s or 40’s.

Forgetfulness and irritability, loss of muscle control, eventually death.

Was brought to US by two brothers from England in 1600’s, who fled allegations of witchcraft. The “strange” behaviors was actually caused by the onset of this disease.

4. Cystic Fibrosis

Recessive autosomalMucus clogs organs, including lungs and liver

1:2,500 whites

5. Tay-Sachs DiseaseRecessiveDeterioration of central

nervous system in infantsCaused by a defective form

of a brain enzyme.1:3,500 Ashkenazi Jews

6. PKU

PKU babies lack an enzyme that converts phenylalanine into the amino acid tyrosine. As a result, phenylalanine build up in the body, causing mental retardation.

Early detection and a special diet can prevent mental retardation.