Genetics The study of heredity

Genetics Genetics is the scientific study of heredity - how traits are passed from generation to generation. The characteristics that are inherited are called traits.

Genes Humans have 23 homologous pairs of chromosomes. On each chromosomes, there are sections called genes that code for traits.

Genes

Alleles An allele is a distinct form of a gene. Every person has 2 alleles for a gene  1 from the father and 1 from the mother

Alleles Letters of the alphabet are used to represent an allele of interest. Every person has two copies of an allele, so they will have two letters. T P A Y R

Alleles Dominant alleles are symbolized with a capital letter. Dominant alleles will mask a recessive alleles in cases of simple dominance/recessiveness. Recessive alleles are symbolized with lower case letters. A a

Homozygous Alleles AA aa If an organism has two like copies of an allele, it is homozygous (homo = same). If the two alleles are dominant, the organism is homozygous dominant. If the two alleles are recessive, the organism is homozygous recessive. AA aa

Heterozygous Alleles Aa If an organism has two different copies of an allele, it is heterozygous (hetero = different). Aa

Genotype and Phenotype The letters an organism has represent the organism’s genotype - what alleles the organism has. As a result of the alleles present, a trait is expressed. The phenotype is the expressed trait.

Genotype and Phenotype Example: In a plant species, there are two alleles for flower color: R and r. R is dominant, and codes for red flowers r is recessive and codes for white flowers

Genotype and Phenotype The genotype is the combination of alleles: either RR, Rr, or rr. The phenotype is what is expressed: either red or white flowers.

Genotype and Phenotype RR - Homozygous dominant Red flowers Rr - Heterozygous dominant rr - Homozygous recessive White flowers In cases of simple dominance, an organism must have two copies of the recessive alleles to express the recessive trait.

Purebreds and Hybrids Purebred - an organism that receives the same genetic traits from both of its parents Hybrid - an organism that receives different forms of a genetic trait (different alleles) from each parent

Contributions of Gregor Mendel Mendel’s Laws Contributions of Gregor Mendel

Law of Dominance The dominant alleles is expressed and may mask a recessive allele. The recessive form of a trait is only shown in a homozygous recessive organism. Ex. R is allele for round, r is allele for square. RR - round Rr - round rr - square

Parent: Dd Parent: dd D d d d Gametes Gametes Law of Segregation Gene pairs separate when gametes are formed. Parent: Dd Parent: dd D d d d Gametes Gametes

Law of Independent Assortment Genes segregate randomly and independently. This means that if there are 2 or more traits, every combination of those traits is possible. AbC Abc abC abc AabbCc

Probability and Punnett Squares Predicting the genotypes and phenotypes of offspring

Probability Probability - the likelihood that a particular event will occur (what are the odds?) What is the probability that a single coin flip comes up heads? 50% or 1/2

Probability True or False? The past outcomes of coin flips greatly affects the outcomes of future coin flips. False. There’s still a 50% chance of heads and 50% chance of tails!

Probability The way in which alleles separate is random, like a coin flip. (Mendel’s Law of Segregation) From a mother who is heterozygous for an allele, there is a 50% chance she passes on the dominant allele and a 50% chance she passes on the recessive allele.

Punnett Squares Punnett squares show probabilities for genotypes and phenotypes of offspring of two parent organisms. Example: In Mendel’s pea plants, the plants had either purple (P) or white (p) flowers.

Punnett Squares Step 1. Make the grid. If there is 1 trait, it is a 2x2 grid. If there are 2 traits, it is a 4x4 grid. Because we are only looking at 1 trait (flower color), a 2x2 grid is needed.

Punnett Squares Pp Step 2: Determine the parents’ genotypes and possible gametes. Example: a heterozygous pea plant and a homozygous dominant pea plant. P p P PP P

Punnett Squares Pp Step 3: Fill in the squares by combining what is on top of the column and to the left of the row. P p P PP Pp PP PP Pp P

Punnett Squares Pp Step 4: Use the Punnett square to determine probabilities and ratios. P p P PP Pp PP PP Pp P

Punnett Squares What is the probability of an offspring plant having purple flowers? 100% What is the probability of an offpsring plant being heterozygous? 2/4 = 1/2 = 50% PP Pp PP Pp

Punnett Squares If there are 2 traits, the Punnett square will be a 4x4 grid. Example: Cross a pea plant that is heterozygous for both flower color and seed shape with a plant that has white flowers and is heterozygous for seed shape P - purple; p - white R - round, r - wrinkled

Punnett Squares PpRr ppRr PR, Pr, pR, pr pR, pr, pR, pr Cross a pea plant that is heterozygous for both flower color and seed shape with a plant that has white flowers and is heterozygous for seed shape PpRr ppRr PR, Pr, pR, pr pR, pr, pR, pr

Punnett Squares ppRr pR pR pr pr PR Pr PpRr pR pr

Punnett Squares ppRr pR pR pr pr PpRR PpRr ppRR ppRr PR Pr PpRr pR pr

Punnett Squares ppRr pR pR pr pr PR Pr PpRr pR pr PpRR PpRr Pprr ppRR

Punnett Squares pR pR pr pr PR Pr pR pr PpRR PpRr Pprr ppRR ppRr pprr 2 / 16 = 1 / 8 or 12.5% Pr pR pr What is the probability of an offspring having white flowers and wrinkled seeds?

Punnett Squares pR pR pr pr PR Pr pR pr PpRR PpRr Pprr ppRR ppRr pprr 6 / 16 = 3 / 8 or 37.5% Pr pR pr What is the probability of an offspring having purple flowers and round seeds?

Punnett Squares PpRR PpRr Pprr ppRR ppRr pprr Write the probable genotypic ratio. 2 PpRR : 4 PpRr : 2 Pprr : 2 ppRR : 4 ppRr : 2 pprr 1 PpRR : 2 PpRr : 1 Pprr : 1 ppRR : 2 ppRr : 1 pprr

Intermediate Inheritance Beyond Simple Dominance

Intermediate Inheritance There are 3 types of intermediate inheritance, genetic patterns that don’t follow the simple dominant-recessive rules. Incomplete dominance Codominance Multiple alleles

Incomplete Dominance Incomplete dominance - neither allele is completely dominant over the other The heterozygous form is a “blended” form of the two alleles.

Incomplete Dominance Example: In snapdragon flowers, there is an allele that codes for red (r), and allele that codes for white (w). rr - red ww - white rw - pink

Incomplete Dominance r w rr rw r r Ex. Cross a red and a pink snapdragon. r w rr rw r r

Incomplete Dominance Sometimes two like capital letters are used, but one gets a prime sign (‘). Ex: Human hair Curly hair HH Straight hair H’H’ Wavy hair HH’

Codominance Codominance - both alleles are dominant and get expressed equally In the heterozygous has some of each phenotype, but they are not blended.

Codominance Example - in a type of cattle, red hair (R) and white hair (W) are codominant. RR - red WW - white RW - roan Some red, some white, but not pink!

Codominance Ex. Cross a red parent and a white parent. R R RW W W

Multiple Alleles Multiple alleles - there are more than 2 alleles for a trait. Ex. Fur color - gray, black, striped Ex. Human blood types

Sex-linked, sex-limited, and sex-influenced traits Sex Linkage Sex-linked, sex-limited, and sex-influenced traits

Human Chromosomes Humans have 23 homologous pairs of chromosomes, for a total of 46. 22 pairs are called autosomes , which are all of the non-sex chromosomes The 23rd pair is the sex chromosomes - X and Y.

Sex Chromosomes X and Y Females - XX All eggs have an X Males - XY Sperm have either an X or Y

Sex-Linked Traits Traits controlled by genes on the sex chromosomes are sex-linked traits. Examples of sex-linked traits: hemophilia, color blindness, male pattern baldness Most are “attached” to the X chromosome. Therefore, females have 2 copies of these alleles and males only have one

Example - Hemophilia Hemophilia - a blood clotting disorder Hemophilia is X-linked. XH = normal Xh = hemophilia Y is still just a Y

Example - Hemophilia Females could be: XHXH - don’t have hemophilia, not a carrier XHXh - don’t have hemophilia, is a carrier XhXh - have hemophilia

Example - Hemophilia Males can be: XHY - does not have hemophilia XhY - has hemophilia Males cannot be carriers - they either have it or they don’t!

Example - Hemophilia XH Y XHXH XHY XHXh XhY XH Xh Draw a Punnett square for cross between a carrier female and an unaffected male. Female: XHXh Male: XHY XH Y XHXH XHY XHXh XhY XH Xh

Example - Hemophilia XH Y XHXH XHY XHXh XhY XH Xh What is the percent chance that a child of theirs will have the disorder? 25% XH Y XHXH XHY XHXh XhY XH Xh

Example - Hemophilia XH Y XHXH XHY XHXh XhY XH Xh What is the percent chance that a child of theirs will have the disorder? 25% XH Y XHXH XHY XHXh XhY XH Xh

Example - Hemophilia XH Y XHXH XHY XHXh XhY XH Xh What is the percent chance that a a son would have the disorder? 50% XH Y XHXH XHY XHXh XhY XH Xh

Example - Hemophilia XH Y XHXH XHY XHXh XhY XH Xh What is the percent chance that a daughter would be a carrier? 50% XH Y XHXH XHY XHXh XhY XH Xh

Example - Colorblindness Color blindness is also X-linked. X = normal Xc = colorblind

Example - Colorblindness Cross a colorblind male and a carrier female. Xc Y XXc XY XcXc XcY X Xc

Sex-limited traits Sex-limited traits are only expressed in the presence of sex hormones, or are only observed in one sex or the other. Ex. Beard growth

Sex-influenced traits Sex-influenced traits are expressed in both sexes, but they are expressed differently. Ex. Baldness is dominant in men, recessive in women

Pedigrees

Pedigrees Males Females Affected - shaded Unaffected - not shaded Carrier - half shaded

Pedigrees A pedigree is a diagram showing family history and tracing a genetic trait.