Chromosomal Basis of Inheritance
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Transcript of Chromosomal Basis of Inheritance
AP Biology
Chromosomal Basis of Inheritance
Chapter 15
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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
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autosomalchromosome
ssex
chromosomes
Classes of chromosomes
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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
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Huh!Sex matters?!
F2generation
100%red-eye female
50% red-eye male50% white eye male
Discovery of sex linkage
P X
F1generation(hybrids)
100%red eye
offspring
true-breeding white-eye male
true-breedingred-eye female
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RR rr
What’s up with Morgan’s flies?x
r r
R
R
Rr
Rr Rr
Rr
100% red eyes
Rr Rr
x
R r
R
r
RR
Rr rr
Rr
3 red : 1 white
Doesn’t workthat way!
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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
X Y
X
X
XX
XY
XY
50% female : 50% male
XX
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XRXR XrY
Let’s reconsider Morgan’s flies…x
Xr Y
XR
100% red eyes
XR
XRXr XRY
XRYXRXr
x
XRXr XRY
XR Y
XR
Xr
XRXr
XRYXRXR
XrY100% red females50% red males; 50% white males
BINGO!
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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)
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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
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Sex-linked usually
means“X-linked”
more than 60 diseases traced to genes on X chromosome
Duchenne muscular dystrophyBecker muscular dystrophy
Ichthyosis, X-linkedPlacental steroid sulfatase deficiencyKallmann syndromeChondrodysplasia punctata, X-linked recessiveHypophosphatemiaAicardi syndromeHypomagnesemia, X-linkedOcular albinismRetinoschisis
Adrenal hypoplasiaGlycerol kinase deficiency
Incontinentia pigmentiWiskott-Aldrich syndromeMenkes syndrome
Charcot-Marie-Tooth neuropathyChoroideremiaCleft palate, X-linkedSpastic paraplegia, X-linked, uncomplicatedDeafness with stapes fixationPRPS-related goutLowe syndrome
Lesch-Nyhan syndromeHPRT-related goutHunter syndromeHemophilia BHemophilia AG6PD deficiency: favismDrug-sensitive anemiaChronic hemolytic anemiaManic-depressive illness, X-linkedColorblindness, (several forms)Dyskeratosis congenitaTKCR syndromeAdrenoleukodystrophyAdrenomyeloneuropathyEmery-Dreifuss muscular dystrophyDiabetes insipidus, renalMyotubular myopathy, X-linked
Androgen insensitivity
Chronic granulomatous diseaseRetinitis pigmentosa-3
Norrie diseaseRetinitis pigmentosa-2
Sideroblastic anemiaAarskog-Scott syndrome
PGK deficiency hemolytic anemia
Anhidrotic ectodermal dysplasia
AgammaglobulinemiaKennedy disease
Pelizaeus-Merzbacher diseaseAlport syndrome
Fabry disease
Albinism-deafness syndrome
Fragile-X syndrome
Immunodeficiency, X-linked,with hyper IgM
Lymphoproliferative syndrome
Ornithine transcarbamylase deficiency
Human X chromosome
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Map of Human Y chromosome?< 30 genes on Y chromosome Sex-determining Region Y (SRY)
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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
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HemophiliaHh x HHXHYXHXh
XHXhXH
Xh
XHYY
XH
sex-linked recessive
XH Ymale / sperm
XH
Xh
fem
ale
/ egg
s XHXH
XHXh
XHY
XhY
XHXH XHY
XHXh XhY
carrier disease
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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
XHXh
patches of black
patches of orange
tricolor catscan only befemale
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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
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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
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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
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As genes are located closer to one another, what happens to their recombination frequency?
Learning Check
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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
B A C
1612
28
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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
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Lab 7: Genetics (Fly Lab)
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Lab 7: Genetics (Fly Lab) Description
given fly of unknown genotype use crosses to determine mode of inheritance of trait
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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
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Lab 7: Genetics (Fly Lab) Conclusions: What is the genotype of the
parent generation?
Case 1
Case 2