AP Biology Chromosomal Basis of Inheritance Chapter 15

AP Biology Morgan & Drosophila  Thomas Hunt Morgan - first to associate a specific gene with specific chromosome in early 20th century  Drosophila breeding  good genetic subject  prolific  2 week generations  4 pairs of chromosomes  3 pair autosomes, 1 pair sex chromosome  XX=female, XY=male 1910 | 1933

AP Biology autosomal chromosomes sex chromosomes Classes of chromosomes

AP Biology Wild Type  Morgan looked for varied traits  Discovered a single male fly with white eyes instead of the usual red  The normal character phenotype is wild type  Alt traits are mutant phenotypes

AP Biology Huh! Sex matters?! F 2 generation 100% red-eye female 50% red-eye male 50% white eye male Discovery of sex linkage P X F 1 generation (hybrids) 100% red eye offspring true-breeding white-eye male true-breeding red-eye female

AP Biology RRrr What’s up with Morgan’s flies? x rr R R Rr 100% red eyes Rr x Rr R r RR Rrrr Rr 3 red : 1 white  Doesn’t work that way!

AP Biology  In humans & other mammals, there are 2 sex chromosomes: X & Y  2 X chromosomes  develop as a female: XX  gene redundancy, like autosomal chromosomes  an X & Y chromosome  develop as a male: XY  no redundancy Genetics of Sex XY X X XX XY 50% female : 50% male XX

AP Biology XRXRXRXR XrYXrY Let’s reconsider Morgan’s flies… x XrXr Y XRXR 100% red eyes XRXR XRXrXRXr XRYXRY XRYXRYXRXrXRXr x  XRXrXRXr XRYXRY XRXR Y XRXR XrXr XRXrXRXr XRYXRYXRXRXRXR XrYXrY 100% red females 50% red males; 50% white males BINGO!

AP Biology Sex-linked Gene  Sex-linked genes appear on X chrom  Females (XX) may have 2 red-eyed alleles or may be heterozygous (all have red eyes)  Males (XY) have only 1 allele and will be red eyed ONLY if they have red-eyed allele (otherwise white allele – white)

AP Biology Genes on sex chromosomes  Y chromosome  few genes other than SRY  sex-determining region  master regulator for maleness  turns on genes for production of male hormones  many effects = pleiotropy!  X chromosome  other genes/traits beyond sex determination  mutations:  hemophilia  Duchenne muscular dystrophy  color-blindness

AP Biology  Sex-linked  usually means “X-linked”  more than 60 diseases traced to genes on X chromosome Human X chromosome

AP Biology Map of Human Y chromosome? < 30 genes on Y chromosome Sex-determining Region Y ( SRY )

AP Biology  Why is crossing over not as common with the sex chromosomes?  What types of genes are on the sex chromosomes?  If there is a gene at the top of the X chromosome, how many alleles does a man have for that gene? Sex- linked gene

AP Biology

Hemophilia Hh x HH XHYXHY XHXhXHXh XHXhXHXh XHXH XhXh XHYXHY Y XHXH sex-linked recessive XHXH Y male / sperm XHXH XhXh female / eggs XHXHXHXH XHXhXHXh XHYXHYXhYXhY XHXHXHXH XHYXHY XHXhXHXh XhYXhY carrierdisease

AP Biology X-inactivation  Female mammals inherit 2 X chromosomes  one X becomes inactivated during embryonic development  condenses into compact object = Barr body  which X becomes Barr body is random  patchwork trait = “mosaic” XHXH XhXh XHXhXHXh patches of black patches of orange tricolor cats can only be female

AP Biology Male pattern baldness  Sex influenced trait  autosomal trait influenced by sex hormones  age effect as well = onset after 30 years old  dominant in males & recessive in females  B_ = bald in males; bb = bald in females

AP Biology 1. If a gene for male pattern baldness is carried on the X chromosome, who in the family would pass the trait to their son? 2. Since females have two X chromosomes and males only have one, women end up with excess genetic info. What does the cell do to make up for this extra amount of DNA? 3. So, if one X chromosome is turned off, why are women not as susceptible as men to sex-linked traits such as color-blindness, balding, MD, and hemophilia? Learning Check

AP Biology  Cell 1- Diploid cell w/ homologous chromosomes. traits A and B are on different chromosomes. Heterozygous.  Chance that they will produce gamete with genotype AB? aB? Ab? Or ab? (50%)  Cell 2- genes A and B are on opposite ends of the same chromosome  Chance that they will produce gamete with genotype AB? aB? Ab? Or ab? (recombination frequency will be 50% due to crossing over)  Cell 3- genes are on the same chromosome close together  Probability of recombination decreases because the genes are less likely to cross over, segregate and independently assort. They will be “linked”. Gene linkage

AP Biology

 As genes are located closer to one another, what happens to their recombination frequency? Learning Check

AP Biology  If gene A recombines 12% of the time with gene B, but 16% of the time with gene C, which gene is closer to gene A?  If gene B and C recombine with one another 28% of the time, what is the order of genes A, B, and C along the chromosome? Recombination frequencies BAC

AP Biology  The following is a map of four genes on a chromosome:  Between which two genes would you expect the highest frequency of recombination?  A) A and W  B) W and E  C) E and G  D) A and E  E) A and G  Learning Check

AP Biology Lab 7: Genetics (Fly Lab)

AP Biology Lab 7: Genetics (Fly Lab)  Description  given fly of unknown genotype use crosses to determine mode of inheritance of trait

AP Biology Lab 7: Genetics (Fly Lab)  Concepts  phenotype vs. genotype  dominant vs. recessive  P, F1, F2 generations  sex-linked  monohybrid cross  dihybrid cross  test cross  chi square

AP Biology Lab 7: Genetics (Fly Lab)  Conclusions: What is the genotype of the parent generation? Case 1 Case 2