Chapter 15 The Chromosomal Basis of Inheritance. The Chromosome Theory of Inheritance Genes have specific loci on chromosomes & it is the chromosomes

Chapter 15

The Chromosomal Basis of Inheritance

The Chromosome Theory of Inheritance

Genes have specific loci on chromosomes & it is the chromosomes that undergo segregation & independent assortment (not individual genes).

We have already seen this in meiosis…

Who figured out that genes made up chromosomes??

A.) The first evidence that genes are located on chromosomes came from the experiments that Thomas Hunt Morgan conducted on fruit flies (Drosophila melanogaster).

1.) Morgan needed a variant fruit flies (with different traits) so that he could trace inheritance patterns.

2.) He bred fruit flies for a year until an individual was born with a variant trait – white eyes instead of the usual red.

I’m awesome.

Who figured out that genes made up chromosomes??

a.) In fruit flies, the “normal” phenotype for a trait is called the wild type and alternatives to this type are called mutant phenotypes.

b.) Symbols used in fruit fly breeding: the allele for white eyes is symbolized by w. A superscript of “+” is for the wild type trait (in eye color, this is red eyes.

i.) So red eyes are w+ and white eyes are just w.

Linking a Gene to a Specific Chromosome

A.) Morgan mated his white-eyed male to a red-eyed female. All F1 offspring had red eyes.

1.) Morgan then mated F1 flies together to produce the F2 generation.

2.) As expected, he got a 3:1 ratio of red to white eyes.

a.) However, flies with white eyes were only MALE!


3.) Morgan inferred that eye color was somehow linked to sex and that the gene for eye color was found on a sex chromosome.

a.) If the eye color gene was found only on the X chromosome and there was no allele

present on the Y, it would make sense that there were no white-eyed females.

Time out for Punnett Squares!

Assuming that eye color is sex linked, let’s draw the Punnett squares representing Morgan’s 1st cross and his cross between two F1 individuals to show how he arrived at his conclusion.


4.) This experiment was the first evidence that genes were found on specific chromosomes. This led to…

5.) Linked genes: genes that are located on the same chromosome and tend to be inherited together.

a.) Morgan began to do many more experiments that provided insight into linked genes showing that genes on the same chromosome are inherited together and crossing over can sometimes separate them.


B.) Morgan’s experiment with body color & wing size:1.) Symbols: b+ = gray, b = black

vg+ = normal wings vg = vestigial wings

Remember, + is the symbol for “wild type.” Black body color and vestigial wings are recessive to the wild types

of gray body color and normal wing size.


2.) Morgan mated true breeding wild type (for both traits) flies (b+ b+, vg+ vg+) with black, vestigial winged flies (bb, vgvg) to produce F1 dihybrids (b+b, vg+vg)

3.) He then crossed females from F1 with males of the “double mutant” phenotype (bb, vgvg)


a.) Do the Punnett square to show the expected results from this cross (this is a dihybrid)!


4.) According to your Punnett square, Morgan should have gotten 25% of offspring that were gray with vestigial wings, 25% gray with normal wings, 25% black with vestigial wings and 25% black with normal wings.

a.) This is assuming that the genes for body color & wing type are located on separate

chromosomes and they assort independently during meiosis.


5.) The actual results Morgan obtained were quite different! Morgan got a much higher % of parental types present in the offspring than he expected.

a.) Parental types: offspring with a phenotype that matches the

phenotype of one of the parents.


6.) , Morgan reasoned that he got these results because the genes for body color & wing size are located on the same chromosome – LINKED to each other!

a.) But, if they are linked, how did he get any offspring with different

combinations of phenotypes? This must be due to crossing over!

Genetic Recombination of Linked & Unlinked Genes

Genetic recombination: production of offspring with combinations of traits different from those found in either parent.

A.) Recombination of unlinked genes is due to independent assortment of chromosomes during meiosis.


1.) Mendel learned from conducting dihybrid crosses that some offspring can have combos of traits that do not match either of the parents’ trait combinations.

a.) Offspring that inherit a phenotype that matches one of the parent’s phenotypes are called parental types.


b.) Offspring that have combos of traits/phenotypes that are different from either of the parents are called recombinants.

c.) If genes for 2 different traits are located on different chromosomes, then in a dihybrid cross, 50% of the offspring should be recombinants.


c.) If 50%, of the offspring are recombinants, we say there is a 50% frequency of recombination.

*d.) So, for any genes that are on different chromosomes, there should be a 50% recombination frequency.


B.) The recombination of linked genes is due to crossing over.

1.) Linked genes should be inherited together because they are on the same chromosome. If they are not (and you get recombinants in the offspring) then crossing over must have taken place between them.


2.) You can figure out the recombination frequency between 2 chromosomes by counting up the number of recombinants among offspring & dividing it by the total # of offspring.


3.) Crossing over is random event & the chance of crossing over occurring is equal at all points along a chromosome.

a.) The farther apart 2 genes are, the higher the probability that a crossover will occur between them & separate them = a higher recombination frequency. In other words, the farther apart two genes are, the higher the recombination frequency.


4.) This information is used to make a linkage map of the relative locations of genes on chromosomes.

5.) The distances between genes on chromosomes are called map units (or centimorgans in honor of Thomas Morgan) with 1 map unit = 1% recombination frequency.

a.) So, if there was a 17% recombination frequency between two genes, we would say they are 17 map units apart on a chromosome.


In summary: When following 2 traits in a dihybrid cross, if the genes for those traits are on different chromosomes, you expect 50% of the offspring to be recombinants.

If less than 50% are recombinants, the genes are linked (on the same chromosome) & the reason for getting any recombinants is crossing over.

We can get the recombination frequency by dividing the total # of recombinants by the total # of offspring & we can use this to map out the relative locations of linked genes on a chromosome (the farther away they are, the higher the recombination frequency).


One exception: Linked genes that are really far away from each other on the same chromosome will almost certainly have a crossover take place to separate them. In this case, their recombination frequency could be 50% which is the same as unlinked genes.

Concept Check (page 282)

Genes A, B and c are located on the same chromosome. Testcrosses show that the recombination frequency between A and B is 28% and between A and C is 12%. Can you determine the linear order of these genes?

Sex-linkage

A.) In humans, the term “sex-linked” refers to genes located on sex chromosomes. It is usually used more specifically to refer to the X chromosome. So, if I tell you a gene is sex-linked, I am implying that it is on the X chromosome.

1.) Genes for some disorders are found on the X chromosome…

a.) Color blindness, hemophilia, muscular dystrophy

X-inactivationA.) Although human females inherit two X chromosomes,

one of those becomes, inactivated during embryonic development in every cell present at that time.

1.) Inactivation is random in each cell – some cells will inactivate the maternal X and some the paternal X.

2.) The inactive X chromosome condenses & lies near the nuclear envelope in each cell. It is now called a Barr body.

a.) Specific genes are active only on the Barr body chromosome to cause its inactivation.

X-inactivation

3.) What does it mean to say that adult females are a “mosaic” of cells due to X inactivation?

a.) Adult females will have 2 types of cells – those with an active paternal X and those with an active maternal X.

Nondisjunction

A.) Nondisjunction: occurs when homologous chromosomes do not separate correctly during meiosis I or sister chromatids don’t separate during meiosis II. Results in gametes that contain an incorrect number of chromosomes.

Nondisjunction

1.) Aneuploidy: condition of having an abnormal number of chromosomes.

a.) If a chromosome is present in 3 copies in a cell, the aneuploid cell is said to be trisomic for that chromosome. If it is present in only one copy, the cell is monosomic for that chromosome.

Nondisjunction

2.) Some organisms can have more than 2 complete sets of chromosomes – this is called polyploidy.

a.) Instead of being diploid (2n) these cells would be triploid (3n) or tetraploid (4n). This is common in plants – can give rise to new species very quickly. For this to happen, there would have to be nondisjunction of ALL chromosomes or a diploid zygote could copy its DNA then fail to divide before copying it again and dividing.

Other Chromosomal Abnormalities

1.) Deletion: chromosomal fragment breaks off & is lost


2.) Duplication: detached chromosomal fragment attaches to, nonsister homologous chromosome thus repeating a segment.


3.) Inversion: detached fragment reattaches but in wrong order.


4.) Translocation: a segment from one chromosome moves to another.

Disorders Caused by Chromosomal Abnormalities

A.) Most aneuploid zygotes do not develop – they are spontaneously aborted (miscarried) by the mother. However, there are some disorders associated with chromosomal abnormalities.

1.) Down syndrome results from trisomy of chromosome 21.


B.) Aneuploidy of sex chromosomes

1.) Klinefelter syndrome: XXY, males who are sterile with small testes and some female development.

2.) Turner syndrome: XO, females with one X chromosome – sterile

3.) XXX and XYY are normal.


C.) Cri du chat syndrome results from a deletion on chromosome 5.

Genomic Imprinting

A.) Genomic Imprinting: variations in phenotype depending on whether an allele is inherited from a male or female parent.

1.) One allele is “silenced” during the formation of gametes in certain genes. That means the resulting zygote will only express one allele of this imprinted gene.

2.) In a given species, imprinted genes are always imprinted in the same way (if the paternal gene is imprinted in one generation, it will be in the next).

3.) Imprinting of some genes seems to be crucial for development.

Extranuclear Genes

A.) These are genes that are located in organelles – mitochondria and chloroplasts contain genes. These organelles reproduce themselves and pass their genes to “daughter” organelles.

1.) Mitochondrial genes are inherited from your mama. The mitochondria present in a zygote arise from the egg cell.

a.) These genes usually code for proteins involved in the ETC & making of ATP.

Questions – pg. 292

1.), Determine the sequence of genes along a chromosome based on the following recombination frequencies: A-B, 8%; A-C, 28%; A-D, 25%; B-C, 20%; B-D, 33%.

2.) The ABO blood type locus has been mapped on chromosome 9. A father who has blood type AB and a mother who has blood type O have a child with trisomy 9 and blood type A. Using this information, can you tell in which parent the nondisjunction occurred? Explain.

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Chapter 15 The Chromosomal Basis of Inheritance. The Chromosome Theory of Inheritance Genes have specific loci on chromosomes & it is the chromosomes