Module 7: Genetics Notes Who is Gregor Mendel? Principle of Independent Assortment: The inheritance of one trait has no effect on the inheritance of another trait. The “Father of Genetics” Contemporary Approaches to Genetics. Module 7. Genetic Notes ©2013 Educurious Partners. All rights reserved. www.educurious.org http://commons.wikimedia.org/wiki/File:Gregor_Mendel.png

Traits Genetics: The study of how traits are passed from parent to offspring. Examples: dimples in your cheek and chin, curly hair, eye color, etc.

Traits are determined by the genes on the chromosomes Traits are determined by the genes on the chromosomes. A gene is a segment of DNA that determines a trait. Might determine your eye color Centromere Might determine your predisposition to Type I Diabetes Image adapted from http://commons.wikimedia.org/wiki/File:Chromosome-upright.png and

Chromosomes come in homologous pairs, so genes come in pairs. Homologous pairs have matching genes – one from the female parent and one from male parent. Example: Humans have 46 chromosomes, or 23 pairs. One set from dad: 23 in sperm One set from mom: 23 in egg

One pair of homologous chromosomes: Gene for earlobe (detached earlobe) Image from http://commons.wikimedia.org/wiki/File:HR_in_meiosis.svg Gene for earlobe (attached earlobe) Alleles: Different genes (possibilities) for the same trait. Example: attached or detached earlobes

Dominant and Recessive Genes A gene that prevents the other gene from “showing” is dominant. A gene that does NOT “show” even though it is present is recessive. Symbols for dominant gene: uppercase letter – T recessive gene: lowercase letter – t Image from http://commons.wikimedia.org/wiki/File:Albino.penguin.bristol.zoo.arp.jpg Dominant color (black and white) Recessive color (white)

Important: Always use the same letter to represent alleles. Example: Straight thumb is dominant to hitchhiker’s thumb T = straight thumb t = hitchhiker’s thumb Important: Always use the same letter to represent alleles. For example, DON’T use S = straight, h = hitchhiker’s https://wikispaces.psu.edu/download/attachments/46924301/Figure+6+Thumbs.jpg?version=1&modificationDate=1241382553000 Straight thumb = TT Straight thumb = Tt *Hitchhiker’s thumb = tt *You must have 2 recessive alleles for a recessive trait to “show.”

Both genes of a pair are the same: homozygous or purebred TT – homozygous dominant tt – homozygous recessive One dominant and one recessive gene: heterozygous or hybrid Tt – heterozygous Images from http://commons.wikimedia.org/wiki/File:Standard_Poodles_Black_and_Brown_Females.jpg BB – Black Bb – Black w/ brown gene bb – brown

Genotype and Phenotype Combination of genes an organism has (actual gene makeup): genotype Example: TT, Tt, tt Physical appearance resulting from gene makeup: phenotype Example: green eyes or not green eyes Images from http://commons.wikimedia.org/wiki/File:Punnett_Square.svg http://commons.wikimedia.org/wiki/File:030608_Pupil.jpg

Punnett Squares and Probability Used to predict the possible gene makeup of offspring: Punnett square Example: Black fur (B) is dominant to white fur (b) in mice Cross a heterozygous male with a homozygous recessive female. Black fur (B) White fur (b) Heterozygous male Homozygous recessive female Images created by Educurious. White fur (b) White fur (b)

Male = Bb X Female = bb Bb b bb B Female gametes: N (One gene in egg) Possible offspring: 2N Male gametes: N (One gene in sperm) Write the ratios in the following orders: Genotypic ratio homozygous : heterozygous : homozygous dominant recessive Phenotypic ratio dominant : recessive Genotypic ratio = 2 Bb : 2 bb 50% Bb : 50% bb Phenotypic ratio = 2 black : 2 white 50% black : 50% white

Now it’s your turn: Cross 2 hybrid mice and give the genotypic ratio and phenotypic ratio: Bb X Bb B b BB Bb bb B b Genotypic ratio = 1 BB : 2 Bb : 1 bb 25% BB : 50% Bb : 25% bb Phenotypic ratio = 3 black : 1 white 75% black : 25% white

Bb X Bb Man = Bb Woman = Bb B b BB Bb bb B b Example: A man and woman, both with brown eyes (B), have a blue-eyed (b) child. What are the genotypes of the man, woman and child? Bb X Bb Man = Bb Woman = Bb B b BB Bb bb B b

Crossing involving 2 traits: Dihybrid crosses Example: In rabbits, a black coat (B) is dominant over brown (b), and straight hair (H) is dominant to curly (h). Cross 2 hybrid rabbits and give the phenotypic ratio for the first generation of offspring. Possible gametes: BbHh X BbHh BH BH Bh Bh bH bH bh bh Gametes BH Bh bH bh BBHH BBHh BbHH BbHh BBhh Bbhh bbHH bbHh bbhh Phenotypes - 9:3:3:1 9 black and straight 3 black and curly 3 brown and straight 1 brown and curly

BBHH X BBHh BH Bh BBHH BBHh Example: In rabbits, a black coat (B) is dominant over brown (b), and straight hair (H) is dominant to curly (h). Cross a rabbit that is homozygous dominant for both traits with a rabbit that is homozygous dominant for a black coat and heterozygous for straight hair. Then give the phenotypic ratio for the first generation of offspring. BBHH X BBHh Possible gametes: BH BH Bh BH Bh Gametes Phenotypes: BBHH BBHh 100% black and straight Gametes (Hint: Only design Punnett squares for the number of possible gametes.)

Sex Determination People: 46 chromosomes or 23 pairs 22 pairs are homologous (look alike) – called autosomes – these determine body traits 1 pair is the sex chromosomes – it determines sex (male or female) Females – sex chromosomes are homologous (look alike): label XX Males – sex chromosomes are different: label XY This is a sequence of a male’s DNA. See the XY chromosome? Image from http://commons.wikimedia.org/wiki/File:NHGRI_human_male_karyotype.png

Incomplete Dominance and Codominance When one allele is NOT completely dominant over another (they blend) the result is incomplete dominance. Example: In carnations, the color red (R) is incompletely dominant over white (W). The hybrid color is pink. Give the genotypic and phenotypic ratio from a cross between 2 pink flowers. RW X RW R W RR RW WW R Image adapted from http://commons.wikimedia.org/wiki/File:Carnations_redoute.JPG W Genotypic = 1 RR : 2 RW : 1 WW Phenotypic = 1 red : 2 pink : 1 white

When both alleles are expressed the result is codominance. Example: In certain chickens, black feathers are codominant with white feathers. Heterozygous chickens have black-and-white speckled feathers. Image from http://commons.wikimedia.org/wiki/File:Speckled_Sussex_Chicken.JPG.jpg

Sex-linked Traits Genes for these traits are located only on the X chromosome (NOT on the Y chromosome). X-linked alleles always show up in males, whether dominant or recessive, because males have only one X chromosome.

Examples of recessive, sex-linked disorders: Colorblindness: the inability to distinguish between certain colors You should see a pink P. Image from http://commons.wikimedia.org/wiki/File:Testcol1.png Color blindness is the inability to distinguish the differences between certain colors. The most common type is red-green color blindness, where red and green are seen as the same color.

XN Xn XNXN XNXn XNY XnY XN Y Example: Give the expected phenotypes of the children of a female who has normal vision but is a carrier for colorblindness, and a male with normal vision. N = normal vision n = colorblindness XN X XN Y XN Xn XNXN XNXn XNY XnY XN Image from http://commons.wikimedia.org/wiki/File:Eye_brown.JPG Y Phenotype: 2 normal vision females 1 normal vision male 1 colorblind male

2. hemophilia: blood won’t clot https://www.youtube.com/watch?v=h2xufrHWG3E&nohtml5=False Image from http://www.nhlbi.nih.gov/health/health-topics/images/hemophilia_01.gif - openly available