Chapter 11 “Mendel and Heredity”

1. Chapter 11“Mendel and Heredity” • Heredity is the passing of traits from parents to their offspring. • The study of heredity is called Genetics. • Gregor Mendel was an Austrian monk who first studied heredity by breeding different varieties of garden peas. • Mendel is considered the “Father of Genetics” because he was the first to develop rules that accurately predict patterns of heredity.

2. Useful Features in Peas • Seven traits that have two clearly different forms that are easy to tell apart. • For example, the flower color is either purple or white- there are no intermediate forms.

3. 2. Mating of the garden-pea flowers are easily controlled- • Both male and female gametes are on the same flower.

4. Mendel carefully cross-pollinated his pea plants by removing the stamen (male reproductive organ that produces pollen) from the flower of one plant. • Then he dusted the pistil (female reproductive organ that produces eggs) of that plant with pollen from a different pea plant

5. 3. The garden pea is small, grows easily, matures quickly and produces many offspring.

6. Mendel’s Work Became a Theory of Heredity • Before Mendel’s experiments, many people thought offspring were just a blend of the characteristics of their parents. • Mendel’s results did not support the blending hypothesis. He correctly concluded that each pea has two separate genes for each trait- one from each parent.

7. Mendel’s Hypotheses • For each inherited trait, an individual has two copies of the gene- one from each parent. • There are alternative versions of genes. For example, the gene for flower color can be purple or white. Different versions of a gene are called alleles. • When two different alleles occur together, one of them may be completely expressed, while the other may have no effect on the organism’s appearance. • The expressed forms of a trait is dominant. • The trait that is not expressed when a dominant trait is present is described as recessive.

8. 4. When gametes are formed, the alleles for each gene in an individual separate independently of one another. • So, gametes only carry one allele for each trait.

9. Genetics • Dominant alleles are written by capitalizing the first letter of the trait. • Recessive alleles are also written using the first letter of the trait but it is not capitalized.

10. If the two alleles of a particular gene present in an individual are the same, the individual is homozygous for that trait. • If the two alleles of a particular gene are different, the individual is heterozygous for that trait. • In heterozygous individuals, only the dominant allele is expressed.

11. The physical appearance of a trait is called a phenotype • The set of alleles that a person has is called the genotype

12. Mendel’s Ideas Gave Rise to the Laws of Heredity • Law of Segregation- during meiosis, the two alleles for a trait separate when gametes are formed.

13. Law of Independent Assortment- the inheritance of one trait does not influence the inheritance of any other trait. • For example, the alleles for plant height separate independently of the alleles for flower color.

14. Studying Heredity • Punnett square is a diagram that predicts the expected outcome of a genetic cross by considering all the possible combinations of gametes in the cross.

15. Monohybrid Cross- a cross that considers one pair of contrasting traits between two individuals. • Use a punnett square to predict the outcome of a cross between a pea plant that is homozygous for yellow seed color (YY) and a pea plant that is homozygous for green seed color (yy).

16. Pedigrees • A pedigree is a diagram that shows how a family trait is inherited over several generations. • They are helpful if the trait is a genetic disorder and the family members want to know if they are carriers. • Carriers are individuals who are heterozygous for an inherited disorder but do not show symptoms of the disorder. • Carriers can pass the allele for the disorder to their offspring.

17. “Squares” = males • “Circles” = females • Horizontal lines = matings • Vertical lines indicate offspring (arranged from left to right in order of their birth). • Colored symbols represent affected individuals

18. Autosomal or Sex-linked? • Autosomal traits will appear in both sexes equally. Autosomes are chromosomes other than an X or Y sex chromosome. • Sex-linked traits are located on the X chromosome. Most sex-linked traits are recessive. • Because males have only one x chromosome, a male who carries a recessive allele on the X chromosome will have the condition.

19. Autosomal dominant traits are expressed if the individual has a parent with the trait. • If the autosomal trait is recessive, one, both or neither of the parents may have the trait. • Do page 343 “Evaluating a pedigree”.

20. Autosomal recessive or autosomal dominant?

21. Complexity of Heredity • Polygenic Traits- several genes influence a trait. • The genes may be scattered along the same chromosome or located on a different chromosome. • Many different combinations appear in offspring.

22. Intermediate Traits • Examples include: eye color, height, weight and hair/skin color. • All have intermediate conditions between one extreme and the other. Blue eyes Blue/black eyes

23. Incomplete dominance • Intermediate traits- a blending of traits from female and male. • Snapdragons example: red flower X white flower = pink flower

24. Codominance • Two dominant traits are expressed at the same time. • A cross between a homozygous red horse and a homozygous white horse results in a heterozygous offspring with both red and white hairs, producing the mixed color roan.

25. Multiple Alleles • Genes with 3 or more alleles. • ABO blood groups (which produces 4 different blood types) are determined by 3 alleles: IA, IB and i. • IA and IB are dominant over recessive i. When both IA and IB are present, they are codominant.

26. Traits influenced by the environment • Examples: soil acidity determines flower color in hydrangeas. • Arctic fox’s fur changes color according to changes in the temperature. • Enzymes make pigments in the summer to darken the coat. • In the winter, no pigment is made so that it can blend with the environment.