Chapter 4 – Biology 11 Textbook Mendelian Genetics Chapter 4 – Biology 11 Textbook

Vocabulary Inheritance Heredity Hybrid Offspring Gene Allele Dominant Recessive Genotype Phenotype Segregation Homozygous Heterozygous Monohybrid cross Dihybrid cross Punnett square Pedigree chart Law of independent assortment Incomplete dominance Codominance

Physical characteristics are inherited Known to be true long before knowledge of genetics Eg: offspring has same colour hair, same shape nose as parent Not limited to humans – inheritance is present in plants and other animals that reproduce sexually Passing of traits from parents to offspring is heredity

Gregor Mendel “father of genetics” Performed experiments on inheritance with pea plants Provided a basis for understanding heredity

Why pea plants? He used cultivated peas for his experiments They grow rapidly Their pollination is easily controlled Pollen grains from the stamen come into contact with egg cells in the ovary They can self-fertilize or be cross-fertilized if self-fertilized: asexual reproduction, offspring are identical to parents If cross-fertilized: sexual reproduction, offspring receives traits from 2 parents

Cross-fertilization

Why pea plants? Many characteristics with 2 forms Seed colour Seed shape Height Pod shape Pod colour Flower colour Flower position Only 2 options – easier to study

Previous theory: Crossing traits would “blend” the traits Eg: smooth + wrinkled = slightly wrinkled Short + tall = medium Yellow + green = yellowish green BUT: Mendel’s experiments proved – no blending Either: smooth OR wrinkled Short OR tall Yellow OR green

Plus… Round seeds + wrinkled seeds = round seeds Round seeds + round seeds = round seeds Wrinkled seeds + wrinkled seeds = wrinkled seeds 1 trait out of the 2 always dominated The other trait is recessive

Genes vs Alleles Genes control inheritance of particular traits Alleles are the alternate forms of a gene

Mendel did not know these terms... Mendel said factors (genes) controlled inheritance & there are different forms (alleles) of each factor Gene: seed colour Alleles: yellow & green 1 allele – dominant, the other – recessive Yellow: dominant over green Round: dominant over wrinkled

Genotype vs Phenotype The dominant allele controls the phenotype Phenotype is the physical characteristic – what you see when the gene is expressed Eg: seed colour, eye colour Both alleles are represented in the genotype Genotype tells you about the genes, specifically what alleles are present Eg: smooth seed has one allele for smooth, one allele for wrinkled OR smooth seed has two alleles for smooth

Genotype, cont’d… Convention: Capital letter represents dominant allele Lowercase letter (same) represents recessive allele So: if yellow seed colour is dominant to green.. Yellow: Y Green: y Plants with both alleles (one from each parent) are hybrids

Mendel’s experiments Cross-fertilization of “pure-breeding” plants Plants that always produced identical offspring Example: he crossed a pure-breeding tall plant with a short plant Characteristics did not blend Only tall plants were produced Tall height is dominant, short height is recessive

The principle of dominance Whenever individuals with opposite characteristics are crossed, offspring express only the dominant characteristic Organism has 2 alleles, but one is recessive and is hidden in the presence of the dominant allele For the recessive allele to be expressed, the 2 alleles must be recessive

So… Phenotype: round seeds Possible genotypes: RR or Rr Phenotype: wrinkled seeds Genotype: rr The genotype is either homozygous or heterozygous

Homozygous: both alleles are the same RR, rr, YY, yy, TT, tt, etc… Phenotype: shows either the dominant or recessive trait Heterozygous: two alleles are different Rr, Yy, Tt, etc… Phenotype: only shows the dominant trait

After Mendel crossed the first generation, then he crossed the hybrids… These are no longer “pure breeding” plants The recessive phenotype showed up again!!!

When 2 hybrid plants were crossed, 75% showed the dominant trait, 25% showed the recessive trait. So: F1 generation were hybrids, Yy, Rr, Tt F2 generation could be pure or hybrids

Possibilities: Tt – genotype, 1 dominant allele for tall, 1 recessive allele for short Tall – phenotype – height of the offspring TT – genotype, 2 dominant alleles for tall Tall – phenotype tt – genotype, 2 recessive alleles for short Short - phenotype

So… Recessive traits are never “lost” when crossed with dominant traits The alleles remain in the population and can be expressed again when combined with other recessive alleles. Let’s take a look at some different examples of monohybrid crosses (testing one allele at a time)

Monohybrid cross

1. Cross between plant that is heterozygous for purple flowers & a plant with white flowers Use a Punnett Square to show the cross & to predict the possible genotypes & their proportions

1. Punnett square: P p Pp pp Phenotypes: 50% of offspring: purple 50% of offspring: white Genotypes: 50% heterozygous purple: Pp 50% homozygous white: pp Punnett square: P p Pp pp

2. Cross between 2 heterozygous tall plants Punnet Square: T t TT Tt Phenotypes: 75% - tall 25% - short Genotypes: 25% homozygous tall: TT 50% heterozygous tall: Tt 25% homozygous short: tt T t TT Tt tt

3. Given: Offspring phenotypes: Yellow seeds: 1578 Green seeds: 526 Work backwards to find the genotypes of the parents.

3. Offspring phenotype: Possible genotypes: Yellow seeds: 1578 Green seeds: 526 Find the ratio: 1578/526 = 3/1 Possible genotypes: Yellow seeds: Yy, YY Green seeds: yy

3. Punnett Square: Yellow Fill in the phenotypes, we know there is a 3:1 ratio of yellow to green Yellow Green

3. Next… We know that the only possible genotype for green is yy, so each parent must have contributed a recessive allele. y Yellow Green yy

3. We also know that each of the yellow offspring must have at least one dominant yellow allele y Yellow Y Green yy

3. Now fill in the rest of the square! We can now see that both parents had heterozygous yellow seeds (Yy) Y y Yellow YY Yy Green yy

4. Homozygous, dominant green pods crossed with a plant with recessive yellow pods Punnett square: Phenotypes: 100% green pods 0 yellow pods Genotypes: 100% heterozygous green: Gg G g Gg

Test Cross Cross that allows the biologist to determine the genotype of an individual for a specific trait Must be crossed with a known (homozygous) recessive individual If any offspring show the recessive trait, the unknown genotype was heterozygous

Pedigree Chart Visual representation of phenotypes within a family Can be used to trace the passing of an allele from parents to offspring

Dihybrid Crosses Laws that apply to a monohybrid cross (one trait) apply to a dihybrid cross (2 traits) Dominant alleles remain dominant, recessive alleles remain recessive Can test for more than one allele at the same time Due to law of independent assortment

Law of Independent Assortment Genes that are located on separate chromosomes are inherited independently of each other So, the expression of one gene does not affect the expression of another Remember independent assortment during meiosis?

Dihybrid cross: example 1 Trait 1: dominant allele: A recessive allele: a Trait 2: dominant allele: B recessive allele: b 2 heterozygous individuals crossed: AaBb x AaBb

Dihybrid cross AB Ab aB ab AABB AABb AaBB AaBb AAbb Aabb aaBB aaBb

Dihybrid cross AB Ab aB ab AABB AABb AaBB AaBb AAbb Aabb aaBB aaBb Phenotypes: 9/16 = 2 dominant traits shown 3/16 = dominant trait A shown, recessive trait B shown 3/16 = recessive trait A shown, dominant trait B shown 1/16 = 2 recessive traits shown

Multiple Alleles It is possible to have more than one allele for a particular gene Increases the complexity of gene expression Therefore, there are more than 2 ways for the gene to be expressed Results in a dominance hierarchy Common examples: eye colour in fruit flies, blood types (A,B,O)

Dominance hierarchy Example: gene with 4 different colour options Colour 1  C1C1, C1C2, C1C3, C1C4 (whenever C1 is present, it is expressed) Colour 2  C2C2, C2C3, C2C4, (C2 is expressed when C1 is not present) Colour 3  C3C3, C3C4 (C3 is only expressed when gene is homozygous, or heterozygous with C4) Colour 4  C4C4 (C4 is only expressed when homozygous) Dominant over Dominant over Dominant over

Examples Cross individual who is homozygous for C2 with individual who has genotype C1C4 Punnett Square: 50% expressing Colour 1, genotype C1C2 50% expressing Colour 2, genotype C2C4 C2 C1 C1C2 C4 C2C4

C3C4 x C1C2 C1C4 x C2C3 Try these on your own.

Incomplete Dominance “Blending” of traits can occur in nature, even though Mendel did not see it in pea plants 2 alleles are equally dominant Intermediate phenotype produced by heterozygous genotype

Incomplete dominance Example: Colour 1 = Colour 2 C1C1 = colour 1 phenotype C2C2 = colour 2 phenotype C1C2 = colour 3 phenotype Eg: snapdragon flower colour Red + white = pink

Codominance Both alleles are expressed at the same time in the offspring Not blended C1C1 = colour 1 phenotype C2C2 = colour 2 phenotype C1C2 = both colours seen in the phenotype Eg: shorthorn cattle

Codominance

Human Blood Types Controlled by multiple codominant alleles Inherit 1 allele from mother, 1 allele from father 6 different genotypes possible: Blood types For simplicity IA A IB B i O

Possible blood genotypes: Allele from Parent 1 Allele from Parent 2 Genotype of offspring Blood types of offspring A AA B AB* AB O AO BB BO OO *A is codominant with B, A & B are dominant over O

Rhesus Factor Rh factor is inherited independently from A, B or O alleles Rh+ or Rh- Finding blood type is a dihybrid cross involving A, B & O alleles & the Rh factor Rh factor Possible genotypes Rh+ Rh+/Rh+ Rh+/Rh- Rh- Rh-/Rh-

To do: p. 147: Case Study: A Mystery Read the evidence, examine the pedigree chart & examine the Blood types of each family member Note that you are told that the murderer is the child of Lord Hooke who is not biologically related to him. Work together in small groups (2 or 3) to complete the analysis questions (a,b,c).

Sex-linked traits Remember: ♀  2 X chromosomes (XX) ♂  1 X, 1 Y chromosome (XY) During meiosis, the XY pair acts as homologous Actually cannot carry the same genes because the Y chromosome is smaller than the X chromosome

Sex-linked traits If a gene is located on the part of the X chromosome that is missing from the Y chromosome… then the ♂ cannot be homozygous for that trait. Phenotype for that gene is determined by the X chromosome e.g. eye colour in fruit flies, red-green colour blindness in humans