Chapter 14 Notes

Mendel and the gene idea

Concept 14.1

In 1857, Gregor Mendel began breeding peas to study inheritance

Geneticists use the term character to describe a heritable feature- ex. flower color

Each variant for a character is called a trait- ex purple flowers or white flowers

Concept 14.1

Mendel began his experiment using true-breeding plants- when the plants self-pollinate, all offspring are of the same variety

Mendel would cross 2 true-breeding plants. This crossing is called hybridization.

Concept 14.1

- the true-breeding parents are the P generation (parental generation) and their hybrid offspring are referred to as the F1 generation (First filial).- when the F1 generation is allowed to self-pollinate, their offspring become the F2 generation (second filial).

Concept 14.1

By the law of segregation, the two alleles for a character are packaged into separate gametes.- P generation: purple x white- F1 generation: all purple

- F2 generation: ¾ purple, ¼ white

Mendel saw this in 6 other varieties

Concept 14.1

Gregor Mendel’s Discoveries

1. Alternative versions of genes (different alleles) account for variations in inherited characters.- the alternative forms of a gene are called alleles.ex. gene: flower color

alleles: white, purple

Concept 14.1

Concept 14.1

2. For each character, an organism inherits two alleles, one from each parent.

3. If the two alleles differ, than one, the dominant allele, is fully expressed in the organisms appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance.

Concept 14.1

4. The two alleles segregate (separate) during gamete production

Punnett squares are used to predict the results of a genetic cross between individuals of known genotypes.

Concept 14.1

Homozygous: when the alleles for a gene are identical

Heterozygous: when the alleles for a gene are different

Phenotype: physical appearanceGenotype: genetic makeup

Concept 14.1

A testcross can be used to determine the genotype of an organism that displays a dominant trait.- ex. Is the purple flower homozygous or heterozygous?

Cross it with a recessive (white) flower. The results will tell the genotype.

Concept 14.1

Concept 14.1

A cross in which only one character is discussed is a monohybrid cross.ex. flower color

If two characters are discussed, the cross is a dihybrid crossex. flower color, seed color

Concept 14.1

Independent law of assortment: each pair of alleles will segregate independently during gamete formation.

Concept 14.3

Incomplete dominance- when the F1 hybrids have an appearance somewhere in between the phenotypes of the parents.- white + red = pink

Concept 14.3

Concept 14.3

Codominance/Multiple Alleles - the two alleles affect the phenotype in separate distinguishable ways. - ex. A, B, AB, and O blood types

Concept 14.3

Concept 14.3

Pleiotropy- the ability of a gene to affect an organism in many ways- ex. the alleles for sickle-cell disease cause multiple symptoms

Concept 14.3

Epistasis- a gene at one locus alters the phenotypic expression of a gene at a second locus- ex. coat color in mice

Concept 14.3

Polygenic inheritance- the additive effect of two or more genes on a single character; not an “either or” situation- converse of pleiotropy where a single gene affects several phenotypic characters- skin color

Concept 14.3

Extending Mendelian Genetics

Nature vs. Nurture: The environmental Impact on Phenotype- the environment plays an important role on one’s phenotype

- ex. nutrition influences height, sun- tanning darkens the skin

Concept 14.3

Norm of reaction: the product of a genotype is not generally a rigidly defined phenotype, but a range of phenotypic possibilities which may be due to environmental influences.

Characters are multifactorial when both genetic and environmental factors influence phenotype

Concept 14.4

Pedigree: a family tree that describes the relationships between generations for a particular trait- pedigrees are used to map out specific phenotypic traits

Concept 14.4

Many human disorders follow Mendelian patterns of inheritance

Recessively Inherited Diseases- if someone is heterozygous for a trait they are termed a carrier. Carriers do not display the trait but can pass on the allele to offspring

Concept 14.4

Common recessive genetic disorders- Cystic fibrosis: 1/2500 people of European decent; 1/25(4%) are carriers- Tay-Sachs disease: - sickle-cell disease: 1/400 African-Americans

Concept 14.4

Common dominant genetic disorders- Anchondroplasia: form of dwarfism- Huntington’s disease: no obvious phenotypic effect until 35 to 45 yrs. old

Concept 14.4