Chapter 8 Mendel, Peas, and Heredity

8-1 Origins of Genetics Heredity – the passing of traits from parents to offspring Genetics – science that focuses of heredity Gregor Mendel – considered the father of genetics Why?

Mendel Austrian monk who used his math and science background to study heredity Used the experiments of T.A. Knight (studied pea plants) Used math to analyze his results (1st to do so) Published his work in 1866

Why Peas? They taste good with butter You can smash them with mashed potatoes Just kidding

Has many traits that come in only two forms Can easily control the mating of plants Small, grows fast, produces many offspring

Mendel’s Classic Experiment His experiment was called a monohybrid cross (cross that involves one pair of contrasting traits) Used plants that were true-breeding in the beginning True-breeding – all offspring would display one form of a specific trait

Step 1 Allowed a group of plants to self-pollinate for several generations Why? To ensure they were true-breeding These plants would serve as the P Generation (Parent) White Flowered TB Plant x Purple Flowered TB Plant

Step 2 Results of P generation cross were all Purple Flowered Offspring Called the F1 Generation Recorded the number of plants expressing the trait

Step 3 Allowed the F1 plants to self-pollinate Resulting offspring called the F2 Generation Most were purple flower plants, some were white flowered plants Why?

8-2 Mendel’s Hypotheses For each inherited trait, an individual has a copy of a gene from each parent There are alternative versions of genes (allele) When two different alleles occur together, one may be completely expressed, while the other is not observable (dominant vs. recessive)

When gametes form, alleles for each gene separate independently of one another (haploid cells)

Homozygous – two of the same alleles (TT or tt) Heterozygous – two different alleles (Tt) Genotype – set of alleles an organism has (TT or Tt or tt) Phenotype – physical expression of a trait (tall or short)

Laws of Heredity Law of Segregation – the alleles of a gene are separated during meiosis

Law of Independent Assortment – States that a random assortment of maternally and fraternally derived chromosomes during meiosis results in gametes that have different combinations of genes Allele for plant height separates from allele for flower color during meiosis

8-3 Studying Heredity Punnett Squares show the probability of a certain cross Remember, not actual results, only possible results

Pedigree Pedigree – a family history that shows how a trait is inherited over several generations Helpful for tracking genetic disorders

Autosomal or Sex-Linked? If a trait is autosomal, it will affect both males and females equally Sex-Linked traits have their allele on the X chromosome Most are recessive Males usually exhibit the trait because they only have one X chromosome. Why?

Females get two chances to get a dominant allele while males only get one allele Females can be carriers of a sex-linked trait (if they are heterozygous)

Rules of Pedigrees Squares = Male Circle = Female Line between male and female means they are married Different generations happen on different levels of the pedigree

Example of a Pedigree This pedigree tracks albinism Albinism is a recessive trait that is autosomal All genotypes are known

Example of a Pedigree This pedigree tracks albinism Only genotype known are those showing the recessive condition

Modes of Inheritance Autosomal Dominant – males and females can be affected, and trait does not skip generations Autosomal Recessive – can affect males and females, and may skip generations

8-4 Patterns of Heredity Most traits are not controlled by dominant vs. recessive alleles When several genes control a trait, they are called polygenic traits Eye color, skin color, height, and weight are all polygenic traits

Incomplete Dominance When an individual is heterozygous for a trait, the individual displays an intermediate form between the two alleles Red Snap Dragon crossed with a White Snap Dragon, and the offspring are pink

Codominance When an individual is heterozygous for a trait, both alleles are expressed at the same time When a homozygous white horsed is crossed with a homozygous red horse, the offspring is roan (red and white hairs)

Multiple Alleles Traits with three or more alleles Human Blood Type has three alleles, IA, IB, and i

Sex Linked Traits Traits whose genes are located on the X or Y chromosome Usually men are more likely to be affected with diseases that are sex linked traits

Environmental Influence Phenotypes often are influenced by the environment Examples of influences include nutrition, social exposure, and temperature

Genetic Disorders Sickle Cell Anemia – autosomal recessive condition caused by abnormal hemoglobin molecules Tay-Sachs – autosomal recessive condition that causes a defective form of a brain protein

Cystic Fibrosis – autosomal recessive condition that causes a defective chloride-ion transport protein Hemophilia A – sex-linked recessive condition that causes a defective blood clotting factor Huntington’s Disease – autosomal dominant condition that causes an inhibitor of brain-cell metabolism