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

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

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

Chromosome Number Each organism on this planet has a specific number of chromosomes Each organism on this planet has a specific number of chromosomes Humans (for example) have 23 pairs of chromosomes with a total of 46 Humans (for example) have 23 pairs of chromosomes with a total of 46 Apes have 24 pairs for a total of 48 Apes have 24 pairs for a total of 48 We share 99% of our genome with chimps and bonobos 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 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 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 Mendel wanted to change the “exact copy” offspring He did so by “cross – pollinating” He did so by “cross – pollinating” He basically pollinated one pea plant with a completely separate one 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 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 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 This led Mendel to believe that there were Dominant traits and less dominant or recessive traits The dominant trait is expressed by a capital letter (for example D) The dominant trait is expressed by a capital letter (for example D) The recessive trait is expressed by a lower case letter (for example d) The recessive trait is expressed by a lower case letter (for example d)

Mendel’s Experiment The original parent pea plants were called the P generation The original parent pea plants were called the P generation TallShort P Generation

Mendel’s Experiment The first set of offspring in the experiment is called the F 1 – Generation The first set of offspring in the experiment is called the F 1 – Generation P Generation TallShortTall F 1 Generation

Mendel’s Experiment The second set of offspring from the F 1 – Generation is called the F 2 - Generation The second set of offspring from the F 1 – Generation is called the F 2 - Generation P GenerationF 1 Generation TallShortTall Short F 2 Generation

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

Genotype vs. Phenotype The Genotype is the “genetic makeup” The Genotype is the “genetic makeup” Homozygous TALL plants would be: T T Homozygous TALL plants would be: T T Heterozygous TALL plants: T t Heterozygous TALL plants: T t Homozygous small: t t Homozygous small: 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 The Phenotype is the “physical expression” of the gene Dominance Rule: Dominance Rule: If a dominant allele is present in the Genotype, the Phenotype will show the dominant trait If a dominant allele is present in the Genotype, the Phenotype will show the dominant trait T is a dominant allele for TALL T is a dominant allele for TALL T T will yield a TALL plant T T will yield a TALL plant T t will also 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)

F 1 Generation Cross Tt x Tt Cross This diagram shows a cross between the F 1 – generation This diagram shows a cross between the F 1 – generation Both plants being crossed are Heterozygous Both plants being crossed are Heterozygous That is; they have both alleles in their genotype That is; they have both alleles in their genotype What would be the outcome? What would be the outcome?

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

Mendel’s Principles Biological characteristics are inherited as genes from parents to offspring Biological characteristics are inherited as genes from parents to offspring Some forms (alleles) of genes are dominant and others are recessive 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 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 The alleles for different genes usually segregate independently from each other

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

Codominance – a case in which both alleles contribute to the phenotype of the organism 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 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 Both alleles are expressed at the same time in the offspring Multiple Alleles – if a gene has more than two alleles Multiple Alleles – if a gene has more than two alleles This means that more than two possible alleles exist in a population This means that more than two possible alleles exist in a population Exceptions to Mendel’s Principles

Multiple Alleles

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