chromosomal basis of inheritance chapter 15. slide 2 of 36 mendel & chromosomes today we know...

Chromosomal Basis of Inheritance

Chapter 15

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Mendel & 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

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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)

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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

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Morgan’s Results

He crossed wild type (red eyes) with a mutant (white eyes), but did not get Mendelian results, or did he?

Gender or sex differences Called Sex-linked gene or trait

ONLY F2 MALES = white eyes !!

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Sex-linked Inheritance

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Now…

Cross the other 3 combinations

What are the other combinations?

Are there any patterns?

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What happens to produce?

All males are mutant, but all females are wild-type?

All females are wild type, but only 50% of males are?

50% are wild-type, 50% are mutant?

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Think about it…

IF the mother is homozygous dominant, then sons are

IF the mother is heterozygous, then sons are

IF the mother is homozygous recessive, then sons are

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Think about it… (Page 2)

IF the father is wild-type, then daughters are

IF the father is mutant, then daughters are

So mothers determine ______ &

wild type fathers produce ______ daughters

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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

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Sex-linked Disorders?

Which gender do you think is afflicted at a higher rate?

Why?

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Hmm?

If XX is female & XY is male, but the Y chromosome contains virtually no genetic material, do females have more genetic information than males?

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Females = X Inactivation

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 BOTH females and males are operating on only 1 sex chromosome

Barr Body – inactivated chromosome condenses Lies on the inside of the nuclear envelope

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More fly stuff

2 Characters: body color & wing size

Body Color

b+ = Grey (wild type)

b = black (mutant)

Wing Size

vg+ = normal wings

vg = vestigial wings (Reduced wing size)

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Vocabulary

Linked Genes – located on same chromosome Tend to be inherited together

Genetic Recombination – Offspring with new combination of genes inherited from parents

Parental Phenotype - at least one of the parental phenotypes

Recombinants – NOT either of the parental phenotypes

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What should have happened?

What should have been the ratio if the characters were inherited via a Mendelian pattern?

How do the recombinants form?

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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)

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Recombination frequency

Calculate by (Total Recombinants / Offspring * 100)

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

More recombinants = less linkage

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Linkage Maps

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

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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.

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Chromosomal Abnormalities

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

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Abnormal NUMBERS of chromosomes

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 2n + 1

Monosomy – only 1 copy of a chromosome 2n – 1

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Abnormal Number of Chromosome SETS

Alteration of an ENTIRE CHROMOSOMAL SET Called polyploidy Triploid = 3n Tetraploid = 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

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Down Syndrome

1 of 700

Trisomy 21 (each cell has 47 chromosomes, not 46)

Risk increases with maternal age

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Klinefelter’s Syndrome

Male have extra X chromosome

Possess male sex organs, but are sterile

IF Female, 3 chromosomes (XXX) = healthy & normal

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Turner Syndrome

Female with only 1 X

Only viable monosomy in humans

chromosomal basis of inheritance chapter 15. slide 2 of 36 mendel & chromosomes today we know...

Documents

daughters slide

wing size slide

smaller size slide

sexlinked inheritance

parental phenotypes

wild type recessive

proper clotting slide

nuclear envelope slide