Unit 7, Part 3 Notes: Sex-Linked Traits and Dihybrid Crosses Pre-AP Biology, Mrs. Krouse

Types of Chromosomes in a Human Cell Describe what is shown in a karyotype. All the chromosomes in a single human body cell (includes the autosomes and sex chromosomes) 2) How many chromosomes are found in a human cell? 46 How many pairs of chromosomes? 23

3) Which pairs are autosomes (pairs 1-22), and which pair is the sex chromosomes (pair 23)? 4) What are the two types of sex chromosomes? X and Y chromosomes LTA

LTB 5) Sex chromosomes found in females (XX)? Males (XY)? 6) Traits found on autosomes? Traits found on sex chromosomes? Autosomes contain genes for body traits that are found in both males AND females Sex chromosomes contain genes for primary or secondary sex characteristics that are only found in males or females

Sex-Linked Traits LTC LTC 9) What does “linked” mean? Synonyms: Related, Connected, To Bring Together In this case, sex-linked traits are traits that are “linked” or connected to genes on the sex chromosomes. In other words, these traits are controlled by genes on the sex chromosomes. 10) X-Linked Traits vs. Y-Linked Traits (see notes) 11-12) Traits on autosomes and X-linked traits have different alleles… what are alleles? Different forms of a gene (ex: purple flower allele vs. white flower allele…. These are different alleles for the flower color gene) LTC

Example of an X-Linked Trait (hemophilia) 12) What is hemophilia? (see notes) 13) Xh = hemophilia allele XH = allele for normal blood clotting

Hemophilia: Possible Genotypes and Phenotypes for Males and Females LTD 14) Genotype Sex Phenotype XH XH Female Normal blood clotting XH Xh Xh Xh Hemophilia XHY Male XhY

15) X-linked recessive traits will be more common in which sex? Why? It is more common in males because… -Males only need to receive one copy of the recessive hemophilia allele (Xh) to have the disease hemophilia -Females must receive two copies of the allele to have the disease… this does not happen a lot LTE

16) Women with the genotype XHXh are called carriers? (see notes)

17) Punnett square reminders… -Why do we use a Punnett square 17) Punnett square reminders… -Why do we use a Punnett square? To predict genotype and phenotype frequencies in the offspring based on the parent genotypes Genotype = combination of two alleles that an organism has for a trait (ex: Aa) Phenotype = physical trait that results from the genotype (ex: Aa  tall) -Letters on top and left are alleles found in gametes. What are gametes? Sex cells (ex: eggs and sperm)

XH Y XHXH XHY Xh XHXh XhY LTsF-I 17) A man who does not have hemophilia marries a woman who is a carrier of the hemophilia allele. What are the genotypes of the parents? XHY and XHXh Set-up and fill in the Punnett square in the space to the right LTsF-I XH Y XHXH XHY Xh XHXh XhY Rules: X comes before Y, and H comes before h Sex and phenotype frequencies in the offspring? (written as percentages) 50% Normal Females 25% Normal Males 25% Hemophilia Males

Tracking the Inheritance of Two Traits LTJ 18-19) Monohybrid vs. dihybrid crosses (see notes) 19) 16 boxes in a dihybrid Punnett square 20) We usually just count the offspring phenotype frequencies

Example Dihybrid Cross Problem 21) Trait 1: G = green leaves and g = yellow leaves Trait 2: P = purple flowers and p = white flowers Parent Genotypes: GgPp x GgPp Rules for Writing Dihybrid Genotypes: -Keep the two alleles for a trait together (keep G’s together and P’s together) -Within a trait, write dominant alleles first (ex: G before g) -Write the alleles in alphabetical order (G before P)

FOIL Method LTL F = first, O = outer, I = inner, L = last 22-24) Use FOIL on the parent genotypes to determine the alleles that go into each gamete on the top and left side of the Punnett square Each gamete should have one allele for each trait (ex: one G and one P) F = first, O = outer, I = inner, L = last Let’s say we wanted to FOIL the parent genotype GgPp First G and P = GP Outer G and P = Gp Inner G and P = gP Last G and P = gp

LTM LTN 25) Let’s set up and fill in our dihybrid Punnett square. GP GGPP GGPp GgPP GgPp GGpp Ggpp ggPP ggPp ggpp 26) Methods for counting offspring phenotype frequencies: color coding, symbol coding, or crossing out box 27) Offspring phenotype frequencies Green, Purple = 9/16 Green, White = 3/16 Yellow, Purple = 3/16 Yellow, White = 1/16 LTN

27) Expressing the offspring phenotype frequencies from the previous slide as ratios… 9 Green, Purple : 3 Green, White : 3 Yellow, Purple : 1 Yellow, White

28) These are offspring phenotype frequencies that we ALWAYS see when doing a dihybrid cross of two parents that are both heterozygous for both traits (ex: GgPp x GgPp) How do we write these frequencies in more general terms? 9/16 with both dominant traits 3/16 with the first trait dominant and the second trait recessive 3/16 with the first trait recessive and the second trait dominant 1/16 with both recessive traits LTO

Another Example Dihybrid Cross Problem 29) Trait 1: R = running mice and r = waltzing mice Trait 2: B = black hair and b = brown hair Parent Information: Let’s cross a homozygous running, heterozygous black mouse with a waltzing brown mouse. 30) What are the genotypes of the two parent mice? Parent 1: BbRR Parent 2: bbrr

31) FOIL the parent genotypes to find the gamete alleles that go on the top and left sides of the Punnett square Parent 1: BbRR (top of square) F = BR O = BR I = bR L = bR Parent 2: bbrr (left side of square) F = br O = br I = br L = br

32) Let’s set up and fill in our dihybrid Punnett square BbRr bbRr 33) Offspring phenotype frequencies as fractions Black, Running = 1/2 Brown, Running = 1/2 33) Offspring phenotype frequencies as a ratio: 1 Black, Running : 1 Brown, Running (This ratio is reduced from a 2:2 ratio)