Mendel and Heredity Source: http://www.hart.k12.ky.us/userfiles/1076/Classes/8647/Bio%20Mendel%20%20Genetics.ppt

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.

Useful Features in Peas Seven traits that have two clearly different forms that are easy to tell apart. (No intermediate forms)

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

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

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

C. Mendel observed that traits are expressed as simple ratios 1 C. Mendel observed that traits are expressed as simple ratios 1. Mendel’s initial experiments were monohybrid crosses. A cross with ONE pair of contrasting traits. 2. Mendel allowed true breeding or self-pollination to occur.

3. These true-breeding plants served as the parental generation in Mendel’s experiments. 4. The parental generation, or P generation are the first two individuals that are crossed.

5. Mendel crossed the P generation to get the F1 generation (First generation). He then examined each F1 plant and recorded the number of F1 plants and their traits. 6. Mendel then mated the F1 generation to get a F2 generation (The second generation) and then again examined each of their traits.

Mendel’s results.

Mendel’s Results and Conclusions Recessive and Dominant Traits Mendel concluded that inherited characteristics are controlled by factors that occur in pairs. In his experiments on pea plants, one factor in a pair masked the other. The trait that masked the other was called the dominant trait. The trait that was masked was called the recessive trait.

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.

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.

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 inherited character or trait.

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.

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.

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

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.

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.

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.

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).

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.

“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

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.

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.

Autosomal recessive or autosomal dominant? AR because neither of the parents have the disorder, but are carriers.

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.

Examples include: eye color, height, weight and hair/skin color. 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

Incomplete dominance Does not support blending hypothesis Snapdragons example: red flower X white flower = pink flower Heterozygous plants have less red pigments

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.

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.

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.