The Chromosomal Basis of Heredity

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Human Chromosomes

• Humans have 46 chromosomes organized as 23 pairs which are homologous because each pair contains the same genes

• Humans are genetically diploid = 2 copies of each chromosome , except for the sex chromosomes (X+Y) which are non-identical

Mammalian Cell Cycle• Cell division cycles occur in stages: - G1 = pre-DNA synthesis - S = DNA synthesis - G2 = post-DNA synthesis - M = mitosis:

cell division occurs by precise steps which distribute one set of chromosomes to each of two cells

Stages of Mitosis

• Occurs in dividing somatic cells

• Chromosome replication:exact duplicates = sister chromatids attached at centromere

• Prophase- chromosomes are visible, spindle fibers organize and attach to chromosomes at kinetochore

Stages of Mitosis• Metaphase- chromosomes line up in center of cell =

metaphase plate• Anaphase - sister chromatids separate: one

member of each pair is pulled to either pole of the cell

• Telophase - nuclei of two new cells reorganize; the cells are diploid = each contains both members of every pair of chromosomes

• Mitosis is usually accompanied by cytokinesis = cytoplasmic division

Meiosis

• Meiosis is a specialized type of cell division which occurs only in reproductive cells = germ cells

• Two rounds of cell division result in the formation of gametes which are genetically haploid = contain only one copy of each pair of homologous chromosomes

Meiosis: First Division

Meiosis occurs in specialized cells called meiocytes in stages and requires two cell division events:

First Meiotic Division: - chromosomes duplicate in S phase - homologous chromosomes pair - homologous chromosomes separate and

are pulled to either pole of the cell at anaphase

Meiosis: Second Division

• Each daughter cell contains only one member of each homologous pair of chromosomes after meiosis I

• Second Meiotic Division: - cell division occurs in the absence of

chromosome duplication - sister chromatids separate at anaphase

as in mitotic division

Meiotic vs. Mitotic Division

• Meiosis produces four cells, each of which contains one copy of each pair of homologous chromosomes = genetically haploid

• Mitosis produces two cells which contain both members of each pair of homologous chromosomes = genetically diploid

Meiosis I: Stages• Prophase - unique process of genetic recombination occurs - homologous chromosomes pair = synapsis

- physical exchange of genetic material occurs between homologous chromosomes

- chiasmata = linkage points

Prophase I: Meiosis

• Leptotene - chromosome condensation• Zygotene - pairing (synapsis) of

homologous chromosomes=bivalent • Pachytene - crossing-over between

homologous chromosomes occurs• Diplotene - chromosome repulsion• Diakinesis- maximum chromosome

contraction

Independent Assortment• Random alignment of homologous chromosomes

during metaphase I results in independent assortment of non-homologous chromosomes

• This occurs because the genetic elements on non-homologous chromosomes are unlinked = inherited as separate physical units

Meiosis I

• Metaphase I - homologous chromosomes line up at metaphase plate; random alignment of non-homologous chromosomes is basis of Law of Independent Assortment

• Anaphase I - physical separation of homologous chromosomes to opposite poles of spindle-demonstrates Law of Segregation;

Meiosis I and II

• Telophase I - spindle breaks down, nuclear reorganization; one homolog from each bivalent is at each pole

• Second Meiotic Division occurs in absence of chromosome replication

• Meiosis II = equational division as the number of chromosomes in each cell remains constant

Meiosis II

• Meiosis II consists of prophase II, metaphase II, anaphase II and telophase II which are identical to the stages of mitosis

• Anaphase II - sister chromatids of each chromosome separate

• Telophase II - each cell contains haploid chromosome number, one member of each homologous pair

Chromosome Structure

• Eukaryotic chromosomes are highly coiled complexes of DNA and protein

• Chromosome size is measured in kb=kilobase pairs; 1 kb=1,000 base pairs; 1 Mb (megabase) = 1 million bp

• Chromosome-sized DNA molecules can be separated by electrophoresis in which DNA molecules move in response to electric field

Chromatin Structure

• Chromatin is a stable, ordered complex of DNA and protein

• Histones = major class of basic proteins in chromatin fibers

• Five major types of histones are found in chromatin:

H1, H2A, H2B, H3 and H4• Histones of different species are similar =

conserved

Chromatin Structure

• Nucleosome = basic structural unit of chromatin

• Each nucleosome core contains about 200 bps of DNA wrapped around a core of histone proteins = two each of H2A, H2B, H3 and H4

• Linker regions with DNA + H1 occur between adjacent nucleosomes

• Structure termed: beads on a string

Chromatin Structure

• Nucleosomes coil to form higher order DNA structure = 30 nm fiber which is a left-handed superhelix or solenoidal supercoil; contains 6 nucleosomes per turn

• 30 nm fiber condenses to compact metaphase chromosome in which DNA/histone complex is attached to scaffold of non-histone proteins

Chromatin Structure

• Heterochromatin = compact, heavily staining chromosome regions rich in satellite DNA and low in gene content

• Euchromatin= less condensed chromosome regions high in gene content

• Satellite DNA = highly repeated non-coding DNA sequences

Chromosomes and Heredity

• Chromosomes consist of linear sequences of genes = genetic information which specifies the physical expression of a phenotypic trait

• Homologous chromosomes contain the same sequence of genes which may vary in expression = alleles

Sex Chromosomes

• X and Y chromosomes = sex chromosomes which are non-identical but share some genes

• Males are genetically haploid for most genes on the X chromosome which results in unique pattern of X-linked inheritance

• Autosomes = non-sex chromosomes

X-Linked Inheritance

• Genes on the X-chromosome are X-linked• Females = XX; Males = XY• Sex of progeny is determined by X or Y of

sperm • Morgan discovered X-linked inheritance

by identifying mutations exclusive to male fruit flies

Morgan’s Fruit Fly Experiments

• Morgan’s studies of inheritance patterns in Drosophila melanogaster revealed important genetic principles

• Fruit flies were excellent tools for research due to short generation time, large number of offspring, and ease of producing and analyzing mutations

Morgan’s Fruit Fly Experiments

Morgan’ s genetic principles:• X-linked inheritance based on mutations

observed in males only

• gene linkage based on the inheritance of genes as a single unit

• chromosome mapping based on recombination frequencies between linked genes

X-Linked Inheritance in Humans

• Many human genes are on the X-chromosome = X-linked

• Males have XY genotype and only one copy of X-linked genes

• Mutations = genetic changes in X-linked genes will be expressed phenotypically in males even if recessive = X-linked genetic disorder

• Hemophilia A =X-linked disorder

Meiosis Error: Nondisjunction

• Nondisjunction = chromosomes fail to separate properly during anaphase of meiosis I or II

• This results in unbalanced chromosome segregation, such that one cell receives both copies of the chromosome pair

• Nondisjunction of X in Drosophila = proof of Chromosome Theory

Nondisjunction

• Nondisjunction in meiosis I produces gametes with a pair of homologous chromosomes

• Nondisjunction in meiosis II produces gametes with a pair of sister chromatids

• Fertilization produces a zygote with 3 copies of a single chromosome = trisomy

Nondisjunction: Aneuploidies

• Nondisjunction can occur for any human chromosome resulting in zygotes with an abnormal number of chromosomes = aneuploidy

• Trisomy of chromosome 21 is the most common autosomal aneuploidy = Down’s Syndrome

• Most aneuploidies are genetic lethals

Mendel’s Ratios: Chi Square Data

Statistical (Chi Square) analysis of Mendel’s experiments in which phenotypic frequencies used to derive phenotypic ratios are the basis for the Law of Dominance, Law of Segregation and Law of Independent Assortment show close correlation between data and predicted outcome

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Did we get the right ratio?

• “Decide” using the Chi-square (2) test• Formula: Observed-Expected)2

Expected• Example -- is 72:28 a 3:1 ratio ????

– step 1: total is 72 + 28 = 100

– step 2: expected is 3/4 x 100 = 75, 1/4 x 100 = 25

– (observed - expected) = 72-75 = -3, squared is 9

– sum over all catagories! (28-25, etc.)_

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Chi-square table

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How we know what to “Expect”

• Statistics

• One tool -- Bionomial Distribution

• Formula: n!s!t!

psqt

– S + t = n (number of trials)– p + q = 1 (probability of two alternatives)– example: probability of 2 boys in 5 sibs– answer = 10/32

• Find factorial term with Pascal’s Triangle

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Pascals Triangle

Chi-Square Analysis

• Goodness of fit = test analyzes whether observed data agree with theoretical expectation

• Statistically significant = refers to the magnitude of the difference between the observed and expected data measurements