HMB265H1 Lecture Notes - Lecture 8: Antonie Van Leeuwenhoek, Walter Sutton, Sister Chromatids

37 views7 pages

For unlimited access to Class Notes, a Class+ subscription is required.

HMB265 Lecture 7 Sex linked traits and the chromosomal theory of inheritance
Evidence that genes reside in the nucleus
- Anton van Leeuwenhoek 1667
o Microscopy revealed that semen contains spermatozoa
o He hypothesized that sperm may enter the egg to achieve fertilization
- 1854 1874
o Direct observations were made of fertilization through the union of nuclei of eggs
and sperm
o Something in the nucleus must contain the hereditary material
- Microscopy provided a means to follow the movement of chromosomes during cell
division
Analysis of chromosomes
- 1903 Walter Sutton studied meiosis in great lubber grasshoppers
- Before meiosis: testes cell had 24 chromosomes
o 22 matched, autosome pairs and two unmatched
- After meiosis: two types of sperm were formed:
o ½ of the sperm had 11 chromosomes and an X
o ½ of the sperm had 11 chromosomes and a Y
- Only one type of egg was produced
o 11 chromosomes and an X
- After fertilization:
o Cells with XX were female
o Cells with XY were males
- Sutton concluded that the X and Y chromosomes determine sex
Chromosomes behavior and inheritance
- Chromosomes carry the units of heredity
- Chromosomes determine the sex of an organism so they must determine other traits as
well
o They contain the basis of inheritance
- Two copies of each kind of chromosome
- Chromosomes complement is unchanged during transmission to progeny
- Homologous chromosomes move to opposite poles and separate to different gametes
o This happens during meiosis
- Fertilization of eggs with sperm occurs randomly
- In all cells derived from a fertilized egg, half of the chromosomes are maternal, and the
other half are paternal
Anatomy of a chromosome
- A chromosome is composed of two sister chromatids
- The sister chromatids are joined at the centromere
- Metacentric chromosome the centromere is in the middle
Unlock document

This preview shows pages 1-2 of the document.
Unlock all 7 pages and 3 million more documents.

Already have an account? Log in
- Acrocentric chromosome the centromere is near the end
- Homologous chromosomes have the same size and genes but may have different
alleles
- Metaphase chromosomes are classified by the position of the centromere
Homologous chromosomes are matched in size, shape and banding patterns
- Homologues contain the same set of genes but can have different alleles
- Nonhomologues carry unrelated sets of genes
- Karyotype is a technique by which chromosomes are arranged in homologous pairs in
decreasing size
- Sex chromosomes unpair X and Y chromosomes
o These chromosomes are unpaired in terms of anatomy and the genes they carry
- Autosomes are all chromosomes except X and Y
- The chromosome number varies with species
o Chromosome number is not correlated to the complexity and size of the species
Mechanisms of sex determination differ between species
- Extremely variable
- Some organisms do not have visible (i.e. heteromorphic) sex chromosomes
- In some organism males are homogametic (ZZ) and females are heterogametic (ZW)
- In some species, gender is determined by the environment
Species
Female
Male
Humans and Drosophila
XX
XY
Moths and C. elegans
XX
(hermaphrodites in C.
elegans)
XO
Birds and butterflies
ZW
ZZ
Bees and wasps
Diploid
Haploid
Lizards and alligators
Cool temperature
Warm temperature
Tortoises and turtles
Warm temperature
Cool temperature
Anemone fish
Older adults
Young adults
Sex chromosomes provide basis for sex determination
- The X and Y chromosomes determine sex in humans
- Children receive one X from the mother and either an X or a Y from the father
- This results in a 1:1 ratio of females : males
Key stages of meiosis and mitosis
1. Mitosis happens in somatic cells
Unlock document

This preview shows pages 1-2 of the document.
Unlock all 7 pages and 3 million more documents.

Already have an account? Log in

Get access

Grade+
$10 USD/m
Billed $120 USD annually
Homework Help
Class Notes
Textbook Notes
40 Verified Answers
Study Guides
1 Booster Class
Class+
$8 USD/m
Billed $96 USD annually
Homework Help
Class Notes
Textbook Notes
30 Verified Answers
Study Guides
1 Booster Class