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. Useful Features in Peas 1. Seven traits that
have two clearly different forms that are easy to tell apart. For example, the flower color is either purple or whitethere are no intermediate forms. 2. Mating of
the gardenpea 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.
Mendels Work Became a Theory of Heredity Before Mendels experiments, many people thought offspring were just a blend of the characteristics of their parents. Mendels results did not support the blending hypothesis. He correctly concluded that each pea has two separate genes for each trait- one from each parent.
Mendels Hypotheses 1. For each inherited trait, an individual has two copies of the gene- one from each parent. 2. 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. 3. When two different alleles occur together, one of them may be completely expressed, while the other may have no effect on the organisms 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 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
Mendels 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. Do page 343 Evaluating a pedigree.
Autosomal recessive or autosomal dominant? 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. 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 Intermediate traits- a blending of traits from
female and male. Snapdragons example: red flower X white flower = pink flower 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 foxs 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.
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