Chromosomal Basis of Inheritance Chapter 15 AP Biology

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

Classes of chromosomes autosomal chromosomes sex 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

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

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

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

Let’s reconsider Morgan’s flies… x X RX R Xr XR XR AP Biology X RX r x X r. Y Y X RY 100% red eyes XR BINGO! Xr X RX r X RY XR Y X RX R X RY X RX r X r. Y 100% red females 50% red males; 50% white males

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

Human X chromosome § Sex-linked Duchenne muscular dystrophy Becker muscular dystrophy usually means “X-linked” u more than 60 diseases traced to genes on X chromosome u Chronic granulomatous disease Retinitis pigmentosa-3 Norrie disease Retinitis pigmentosa-2 Hypophosphatemia Aicardi syndrome Hypomagnesemia, X-linked Ocular albinism Retinoschisis Adrenal hypoplasia Glycerol kinase deficiency Ornithine transcarbamylase deficiency Incontinentia pigmenti Wiskott-Aldrich syndrome Menkes syndrome Sideroblastic anemia Aarskog-Scott syndrome PGK deficiency hemolytic anemia Anhidrotic ectodermal dysplasia Agammaglobulinemia Kennedy disease Pelizaeus-Merzbacher disease Alport syndrome Fabry disease Immunodeficiency, X-linked, with hyper Ig. M Lymphoproliferative syndrome Albinism-deafness syndrome Fragile-X syndrome AP Biology Ichthyosis, X-linked Placental steroid sulfatase deficiency Kallmann syndrome Chondrodysplasia punctata, X-linked recessive Androgen insensitivity Charcot-Marie-Tooth neuropathy Choroideremia Cleft palate, X-linked Spastic paraplegia, X-linked, uncomplicated Deafness with stapes fixation PRPS-related gout Lowe syndrome Lesch-Nyhan syndrome HPRT-related gout Hunter syndrome Hemophilia B Hemophilia A G 6 PD deficiency: favism Drug-sensitive anemia Chronic hemolytic anemia Manic-depressive illness, X-linked Colorblindness, (several forms) Dyskeratosis congenita TKCR syndrome Adrenoleukodystrophy Adrenomyeloneuropathy Emery-Dreifuss muscular dystrophy Diabetes insipidus, renal Myotubular myopathy, X-linked

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

Sex- linked gene § 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? AP Biology

AP Biology

sex-linked recessive Hemophilia HX h x X HY HH XHh XH female / eggs male / sperm XH Xh AP Biology XH Y X HX H X HY X HX h Xh XH X HX h X h. Y carrier disease X HY Y

X-inactivation § Female mammals inherit 2 X chromosomes u one X becomes inactivated during embryonic development § condenses into compact object = Barr body § which X becomes Barr body is random w patchwork trait = “mosaic” patches of black X HX h tricolor cats can only be AP Biology female Xh patches of orange

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

Learning Check 1. If a gene for male pattern baldness is 2. 3. carried on the X chromosome, who in the family would pass the trait to their son? 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? 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? AP Biology

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

AP Biology

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

Recombination frequencies § 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? B A C 12 AP Biology 16 28

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

Lab 7: Genetics (Fly Lab) AP Biology

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

Lab 7: Genetics (Fly Lab) § Concepts phenotype vs. genotype u dominant vs. recessive u P, F 1, F 2 generations u sex-linked u monohybrid cross u dihybrid cross u test cross u chi square u AP Biology

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