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Lecture 13

01:119:115 Lecture Notes - Lecture 13: Embryology, Wild Type, Mendelian Inheritance

5 pages105 viewsFall 2018

Biological Science
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Lecture 13
Review: Meiosis and Genetics
- Mendel didn’t know about genes and chromosomes
- 1902: chromosomal theory of inheritance
- Fig. 15.2
I. Chromosomal Theory of Inheritance : genes have specific loci (locations) on
chromosomes. Chromosomes undergo segregation and independent assortment
How did we figure this out? Why can’t each gene behave independently like
Mendel thought?
A. Thomas Morgan and D. melanogaster
1. Early 20th century, experimental embryologist
2. 1860s - 1900s: cytologists start seeing that chromosomes behave like
Mendel’s “heritable factors”
3. Originally a skeptic of Mendel’s work and chromosomes theory
4. Significance:
a) 1st experimental support for genes on chromosomes
b) Chromosomal theory fit what was known about Mendel’s “heritable
c) Drosophila melanogaster : a fruit fly; Morgan’s model organism
(1) Advantages:
(a) Lots of offspring
(b) Short generations: ~2 weeks
(c) 4 pairs of chromosomes - 3 pairs of autosomes, 1
pair of sex chromosomes
5. Describing Traits
a) Wild Type : most common phenotype for a character in natural
(1) Ex.: red eyes
b) Mutant Phenotype : alternatives to wild type
(1) Ex.: white eyes
6. Notation
a) Conventions differ
b) In Drosophila, gene gets symbol based on 1st observable mutant
Allele for white eyes = w
Allele for red eyes = w+
w+ indicates wild type
c) Wild type ≠ dominant
(1) Ex.: for humans, having six fingers is dominant, but our
wild type is five fingers
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B. Correlating Allelic and Chromosome Behavior
1. P: red-eyed female x white-eyed male
a) F1: All offspring have red eyes
→ Red eyes are dominant over white eyes
b) Red-eyed female F1 xred-eyed male F1 F2 3:1 phenotypic
(1) All the white-eyed flies are male - no white-eyed females
2. Sex Determination Explains Ratio
a) Y chromosomes does not = male
b) Y chromosome → make sperm
c) Ratio of X chromosomes to sets of autosomes (A) determines
(1) XX:AA → 1:1 → female
(2) XY:AA → 1:2 → male
(3) XO:AA → 1:2 → sterile male
(4) XXX:AAA → 1:1 → female
(5) XXY:AA → 1:1 → female
d) Why no white-eyed females?
(1) Eye-color gene is located on the X-chromosome; there is
no corresponding locus on Y-chromosome
(2) Eye-color Monohybrid Cross
(a) P: Xw+Xw+ x XwY
(b) F1: Xw+Xw females, Xw+Y males
(c) F1 cross: Xw+Xw x Xw+Y.
(d) F2:
(i) Xw+Xw+ = female red
(ii) Xw+Xw = female red
(iii) Xw+Y = male red
(iv) XwY = male white : only males have white
3. Morgan’s Findings
a) Specific genes are carried on a specific chromosome
b) Unique inheritance patterns for genes on sex chromosomes
c) Strong support for chromosome theory of inheritance
II. Inheritance Patterns on Sex Chromosomes
A. Sex Chromosomes
1. 1 pair
2. Sex-determining genes
3. Heteromorphic (differ in size, shape, structure, etc)
4. Act homologous during meiosis
5. Contain genes unrelated to sex determination
6. Sex-linked genes : genes on either sex chromosome
7. Human Sex Determination
a) Males - XY → heterogametic
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