CHAPTER 15. You Must Know How the chromosome theory of inheritance connects the physical movement of chromosomes in meiosis to Mendels laws of inheritance.

CHAPTER 15

You Must Know

How the chromosome theory of inheritance connects the physical movement of chromosomes in meiosis to Mendel’s laws of inheritance

The unique pattern of inheritance in sex-linked genes

How alteration of chromosome number or structurally altered chromosomes (deletions, duplications, etc.) can cause genetic disorders

Linking Mendel and Chromosomes Today we know that Mendel’s “hereditary

factors” are located on chromosomes So we can link Mendelian genetics to modern

genetics through the genes that lie on the chromosome

Chromosome Theory of Inheritance Mendelian genes have specific loci (positions) on

chromosomes Chromosomes undergo Segregation &

Independent Assortment

T.H. Morgan studied flies

He studied Drosophila Melanogaster – Fruit fly They _____ like flies Take 2 weeks to breed Hundreds of offspring per brood Only 4 pairs of chromosomes

3 autosomes and 1pair of sex chromosomes

Converting to Morgan speak

Phenotype Dominant = wild type Recessive = mutant

Wild-type – normal or typical W+

Mutant type – not normal W

For example, vg+ = wild type for body size (full body)

vg = mutant (vestigial body, smaller size)

Morgan gets unMendelian results He crossed wild type (red eyes) with a

mutant (white eyes), but did not get Mendelian results, or did he?

Basis for his questioning Mendelian Genetics

P = Red-eyed female + White-Eyed male F1 = ALL Red eyes F2 = 3:1 ratio of red to white eyes, BUT

only MALES = white eyes

Gene Linkage

Linked genes – genes located on the same chromosome that tend to be inherited together

Linked genes do not adhere to the laws of Mendelian genetics

Sex-linked gene inheritance pattern

Notes on Sex-linked Traits

Female = XX Male = XY

Fathers pass sex-linked traits to daughters NOT sons

Fathers pass the y-chromosome to sons Female expression is like any other trait Since Males have only 1 chromosome, they

express the allele on their 1 X-chromosome

Sex-linked traits have NOTHING to do with sex CANNOT use heterozygous and homozygous

Known Sex-linked Disorders

Duchenne muscular dystrophy Progressively weakening of muscles and loss of

coordination

Hemophilia Blood that is unable to clot normally Due to absence of proteins required for proper

clotting

X Inactivation in Females

Although females receive 2 copies of alleles, one chromosome becomes inactivated during embryonic development

Due to XX Chromosome inactivation is Random Inactivation is due to methylation So females and males are operating on only

1 copy Barr Body – inactivated chromosome

Lies on the inside of the nuclear envelope

Mosaic-ness

Back to Morgan & Flies

Linkage Inheritance Pattern

2 Characters: body color & wing size Body Color

b+ = Grey (wild type)

b = black (mutant)

Wing Sizevg+ = normal wings

vg = vestigial wings (Reduced wing size)

Some more vocab…

Linked Genes – located on same chromosome

Tend to be inherited together Genetic Recombination – Offspring with

new combination of genes inherited from parents

Parental Phenotype – offspring phenotype = at least one of the parental phenotypes

Recombinants – offspring phenotype NOT = either parental phenotype

Notes on the Results

2,300 offspring Far higher proportion of parental

phenotypes than expected from independent assortment

Genes are inherited together There were also recombinants or non-

parental phenotypes as well Conclusion = Partial linkage & Genetic

recombination (recombinants or recombinant types)

Recombinant Frequency

If the genes are located on different chromosomes, then the recombination frequency should be 50%

In the flies, the recombinant frequency was less than 50%; it was about 17%

Evidence of that the 2 genes lied on the same chromosome

So some linkage but incomplete

Crossing Over

Crossing Over explains why some linked genes get separated during meiosis

Crossing Over occurs in Meiosis I

Farther apart 2 genes = Higher P(Crossing Over)

Linkage Map – genetic map based on the percentage of cross-over events

Map unit – 1% recombination frequency Used ONLY for relative distances on the chromosome

Linkage Maps

Explain

We know that Mendel’s seed color and flower color were on the SAME chromosome, but they did not behave as linked genes. Explain.

Nondisjunction

Nondisjunction – mishap where pairs of homologs do not move apart properly during meiosis

Could happen in Meiosis I or when Sister chromatids fail to separate correctly in Meiosis II

One gamete receives 2 of the same type of chromosome, while another receives no copy

-somy

Aneuploidy – abnormal number of chromosomes

Nondisjunction could result in a cell with 2n+1 Here this cell would be considered aneuploid, and

considered trisomic for that individual chromosome

-somy = different number of an INDIVIDUAL chromosomes

Trisomy – 3 copies of a chromosome Monosomy – only 1 copy of a chromosome

2n – 1

-ploidy

Alteration of an ENTIRE CHROMOSOMAL SET Called polyploidy Triploidy = 3n Tetraploidy = 4n Polyploidy plants are fairly common,

animals are less common Polyploids are more normal than aneuploids

Hence, 1 chromosome extra or fewer is more disruptive, than an entire set of chromosomes extra or fewer

Chromosomal Structural Alteration

Human Disorders caused by Chromosomal Alteration

Down Syndrome 1 of 700 Trisomy 21 (each cell has 47 chromosomes, not 46) Risk increases with maternal age

Klinefelter Syndrome Male have extra X chromosome Possess male sex organs, but are sterile IF Female, 3 chromosomes (XXX) = healthy &

normal Turner Syndrome

Female with only 1 X chromosome Sterility, but only known monosomy in humans