Mendel and Heredity Chapter 8 Read the section reviews.

Mendel and Heredity

Chapter 8

Read the section reviews

The Origins of Genetics

• Heredity is the passing of traits from parents to offspring.

• Gregor Mendel did experiments with pea plants on the passing of seven traits.

• He discovered patterns in the passing of traits and these patterns are called genetics.

• Mendel’s parents were peasant farmers and he was a priest and teacher.

• Mendel used an experiment of T.A. Knight but kept records.

Useful Features in Peas

• Characteristics occur in two distinct forms.– Flower color is a character, purple & white are traits of

the flower color character.

• Pea plants self-fertilize or make clones.– To cross-pollinate remove the stamen of one plant

and transfer pollen form another stamen

• Peas are small, grows & matures quickly, and produces many offspring.– This gives quick results that are easy to count.

• See Table 1 on page 163.

Monohybrid Cross

• Monohybrid cross involves crossing two plants with one pair of contrasting traits.

• To ensure true-breeding plants self-pollinated for several generations.

• The parents are the P Generation• The offspring of the P generation is the

First Filial or F1 Generation• A cross of two F1 plants gives the F2 or

second Filial generation.

Mendel’s Results

• The P generation was ½ purple & ½ white.• In the F1 generation one trait disappeared

in all of the plants. (All purple, no white)• In the F2 generation the white trait

reappeared in ¼ of the plants.• The ratio of purple to white flowers was

3:1.• This pattern was the same for all 7

characteristics.

Mendel’s Theory

• Before Mendel people believed in blending

• Mendel saw “heritable factors” were passed to offspring from parents.

• The father passed one of his two “factors” as did the mother to the baby.

• When gametes fuse to make a zygote the “factors” are combined.

Mendel’s Theory

• Each individual has two copies of a character, one from each parent.

• A character is now called a gene.• There are alternate versions of the genes are

called Alleles. purple, white are alleles• One Allele is dominant & makes protein.• One allele is recessive & doesn’t make protein.• Genes making protein are called expressed.

Gametes and Alleles

• Mendel said when gametes form in a plant the alleles from that plants parents segregate independently of each other.

• Each gamete has one allele in it.

• When the gametes unite the baby will have two alleles for each trait. One from the mother and one from the father.

Genetic Vocabulary

• The physical appearance of an organism is its phenotype. (purple flowers)

• The combination of alleles is the genotype. (PP, Pp, pp)

• If an organism has the same alleles in its genotype it is called Homozygous. (TT)

• If an organism has different alleles for a trait it is called Heterozygous. (Tt)

Laws of Heredity

• Law of Segregation: states that when gametes form (meiosis) the two alleles a person has separate into different gametes.

• Law of Independent Assortment: states that the segregation of one pair of alleles has no affect on the segregation of a different set of alleles.

Studying Heredity

• Scientists use Punnett Squares to predict the possible outcomes of breeding parents

• Punnett Squares have four boxes with the alleles of one parent on top and the alleles of the other parent on the side. The alleles passed to the offspring are inside the boxes.

• Phenotypic and genotypic ratios are determined from the Punnett Squares.

Test Crosses

• A test-cross crosses an unknown genotype with a homozygous recessive (tt)

• If no recessive shows then the unknown was homozygous dominant, if the recessive shows the unknown was heterozygous.

• This cross will determine the genotype of an unknown organism.

Probability of a Cross

• Probability is the chance a specific event will occur. (Coin Flip heads to tails)

• Probability= # of 1 kind of outcome / total # of possible outcomes

• Probability of an allele in a gamete is ½• Probability of a cross is determined by

multiplying the individual events. • Flip two different coins, four possible

outcomes. Same for alleles in parents.

Pedigrees & Sex-linked Genes

• A pedigree is a diagram that shows history of a trait through many generations.

• Genes on the Sex chromosomes (XY) are sex linked. Genes on the autosomes (1-22) are not linked to a sex.

• Sex-linked disorders are more common in males than females. Females have 2 X’s

Complex Patterns of Heredity

• Incomplete Dominance- occurs when a single dominant does not make enough protein to mask the recessive.

• Codominance- occurs when two alleles both make their protein.

• Multiple Alleles- occurs when there are more than two alleles. (Human Blood type)

• Polygenetic Traits- occurs when a trait has more than one location

• Environmental conditions can alter proteins made by the genes.

Figure 11-11 Incomplete Dominance in Four O’Clock Flowers

Genetic Disorders• Genes maybe copied incorrectly or damaged

causing proteins not to function.• Sickle Cell Anemia has hemoglobin made

incorrectly and red blood cells change shape when lacking oxygen.

• Cystic Fibrosis has a defective gene to move mucus from cells.

• Hemophilia is on the X chromosome and does not allow blood to clot. (AHF protein)

• Huntington’s Disease is dominant but does not take affect until later in life after it is passed to the kids.

Gene Therapy

• Genetic Counseling will determine the risks an individual with genetic disorders in the family to pass the gene to their kids.

• Gene therapy uses viruses to inject working genes into cells with nonworking copies.