Mendelian Genetics

Mendelian Genetics Genetics Lecture III

Biology Standards Covered • 2c ~ students know how random chromosome segregation explains the probability that a particular allele will be in a gamete • 2e ~ students know why approximately half of an individual’s DNA sequence comes from each parent • 2g ~ students know how to predict possible combinations of alleles in a zygote from the genetic makeup of the parents

Biology Standards Covered • 3a ~ Students know how to predict the probable outcome of phenotypes in a genetic cross from the genotypes of the parents and mode of inheritance (autosomal or x-linked, dominant or recessive) • 3b ~ Students know the genetic basis for Mendel’s laws of segregation and independent assortment

Gregor Mendel • Thought of as the “founder” of modern genetic research • He worked on an Austrian monastery in the mid 1800’s • His work with the “garden peas” explained many unanswered questions about genetics

Important Vocabulary • Heredity – the passing on of characteristics from parents to offspring • Traits – characteristics that are inherited (for example your eye color) • Genes – chemical factors that determine traits

Important Vocabulary • Gametes – individual male and female sex cells • Pollination – the transfer of pollen (male gametes) to the pistil (female part of a flower) • Fertilization – the uniting of male and female gametes

Chromosome Number • Each organism on this planet has a specific number of chromosomes • Humans (for example) have 23 pairs of chromosomes with a total of 46 • Apes have 24 pairs for a total of 48 • We share 99% of our genome with chimps and bonobos

Garden Pea plants • The garden pea plants were “self pollinating” when Mendel first observed them • This means that the offspring of each pea plant was an exact copy of the parent plant

Garden Pea plants • Mendel wanted to change the “exact copy” offspring • He did so by “cross – pollinating” • He basically pollinated one pea plant with a completely separate one

Genes and Dominance • Mendel found that when you make certain “crosses” some of the traits were hidden or masked • When he made further crosses some of these traits came back

Genes and Dominance • This led Mendel to believe that there were Dominant traits and less dominant or recessive traits • The dominant traitis expressed by a capital letter (for example D) • The recessive traitis expressed by a lower case letter (for example d)

Mendel’s Experiment • The original parent pea plants were called the P generation P Generation Tall Short

Mendel’s Experiment • The first set of offspring in the experiment is called the F1 – Generation P Generation F1 Generation Tall Short Tall Tall

Mendel’s Experiment • The second set of offspring from the F1 – Generation is called the F2 - Generation F2Generation P Generation F1 Generation Tall Short Tall Tall Tall Tall Tall Short

Alleles • Alleles are different forms of a gene • By “form” we mean the amount or variety of traits • If there are three different possible colors for flowers on a pea plant, then there are three possible Alleles • For example: • a TALL allele may be written as T • a short allele may be written as t

Genotype vs. Phenotype • The Genotype is the “genetic makeup” • Homozygous TALL plants would be:T T • Heterozygous TALLplants:T t • Homozygoussmall: t t *The recessive trait is “short” and in order for the plant to be short both alleles must be lowercase (recessive)

Genotype vs. Phenotype • The Phenotype is the “physical expression” of the gene • Dominance Rule: • If a dominant allele is present in the Genotype, the Phenotype will show the dominant trait • Tis a dominant allele for TALL • T T will yield a TALL plant • T t will also yield a TALL plant * The ONLY way a short plant will occur is if BOTH alleles are recessive!! (tt)

F1 Generation Cross TtxTtCross • This diagram shows a cross between the F1 – generation • Both plants being crossed are Heterozygous • That is; they have both alleles in their genotype • What would be the outcome?

Probability in Genotypes • In a Heterozygous cross • 1 out of 4 will be homozygous dominant (25%) • 2 out of 4 (1/2) will be heterozygous dominant (50%) • 1 out of 4 will be homozygous recessive (25%)

Mendel’s Principles • Biological characteristics are inherited asgenes from parents to offspring • Some forms (alleles) of genes are dominant and others are recessive • In most sexually reproducing organisms, each adult has two copies of a gene – one from each parent • The alleles for different genes usually segregate independently from each other

Exceptions to Mendel’s Principles • Some alleles are neither dominant nor recessive • Many traits are controlled by multiple alleles or genes • Incomplete Dominance – when there isn’t a dominant allele at all • This appears as a third allele (red, white, & pink)

Exceptions to Mendel’s Principles • Codominance– a case in which both alleles contribute to the phenotype of the organism • If a red and white flower were crossed, codominance would result in a red flower with white stripes or white with red stripes • Both alleles are expressed at the same time in the offspring • Multiple Alleles – if a gene has more than two alleles • This means that more than two possible alleles exist in a population

Multiple Alleles

Exceptions to Mendel’s Principles • Polygenic traits – when a trait is controlled by two or more genes that can be found on similar or completely different chromosomes • Human skin color is controlled by more than four individual genes • Human eye color is also considered polygenic

Mendelian Genetics