13.1 Chromosome Theory and Genetic Linkage 1902 - embryonic development of sea urchins did not occur unless chromosomes were present Meiosis observed using a microscope, chromosomes separated into daughter cells. Lead to the CHROMOSOMAL THEORY OF INHERITANCE - chromosomes are the genetic material responsible for Mendelian inheritance.

When Mendel crossed his F1 generation: RrYy x RrYy, he got a 9:3:3:1 ratio This ratio does not hold true when genes are located on the same chromosome INDEPENDENT ASSORTMENT

A return to Drosophila.... Thomas Hunt Morgan studied fruit flies and found that in some crosses, expected outcomes weren't happening. In this cross GgBb x ggbb, he did not get a ¼ , ¼ , ¼, ¼ ratio.

What is up with all those letters? or just use the old fashioned way GgB b x gg b b

Because these alleles are found on the same chromosome, a Punnett square would show them inherited together. There are two possible arrangements for the heterozygote (AaBb) in the above cross. If the dominant alleles are on different chromosomes (Ab) then it is called TRANS If the dominant alleles AB are on the same chromosome, it is called a CIS arrangement

Consider a cross with a parent that has the CIS arrangement to one that is a mutant for both traits (aabb) A B a b a b a b X 50% wild 50% mutants

Thomas Hunt did not observe a perfect 1:1 ratio Thomas Hunt did not observe a perfect 1:1 ratio. Instead, his results looked like this…. Expected Observed Wild Type 50 40 Mutant Vestigial wings, Wild 10 Black body, Wild Question: How would you explain these results?

Answer: The two offspring that did not look like either parent are called recombinants. They are a result of CROSS-OVER which occurred during meiosis, the alleles switch position.

Using this methodology, the chromosomes of the fruit fly were mapped Using this methodology, the chromosomes of the fruit fly were mapped. Each MAP UNIT represents how far apart the alleles are on the chromosome, the number is based on how often crossing over occurs.

Practice Questions (assume no crossing over occurs) 1 Practice Questions (assume no crossing over occurs) 1. A dumpy winged (dd) fruit fly with long aristae (AA) is crossed with a long winged (Dd) short aristae (aa). Show the cross and the phenotypic proportions.

2. A fruit fly with short legs (ll) and vestigial wings (ww) is crossed with one that is heterozygous for both traits. Assuming the dominant alleles are on separate chromosomes, show the cross and the expected phenotypic proportions.

3. A purple eye fly (pp) with vestigial wings (gg) is crossed with one that is heterozygous for both traits, cis arranged. What phenotypes are expected in the offspring?

4. Chromosome Map Problem In pea plants, flower color and pollen shape are located on the same chromosome. A plant with purple flowers and long pollen (AaBb) is crossed with one that is recessive for both traits (aabb). The results are as follows: Results Purple, long 47 Red, round Purple, round 3 Red, Long a) Are the chromosomes of the AaBb parent in the cis or trans position? Sketch the punnett square showing the expected offspring. b) How are apart are the two alleles?

13.2 Chromosome Disorders Karyotypes (karyograms) are used to determine problems with chromosomes or their numbers. Normal = 22 pairs of autosomes, 2 sex chromosomes Caused by nondisjunction of chromosomes during meiosis Down Syndrome = Trisomy of chromosome 21 Klinefelter Syndrome = XXY (male) Turner Syndrome = XO (monosomy, female)

Trisomy 18 - Edward's Syndrome Failure to grow and gain Developmental delays and intellectual disability A prominent back portion of the head,abnormally small jaw, a small mouth an upturned nose, narrow eyelid folds

TRISOMY 13 (also known as Patau syndrome) - Of all babies born with the extra copy of chromosome 13 in all the cells of their body, around 50% die in the first month, and the rest within the first year Median survival age for children with Patau syndrome is 2.5 days

Structural Problems with Chromosomes Deletion - section of a chromosome is missing Cri-du-Chat syndrome is deletion of the 5p arm of chromosome 5 Cat Eye Syndrome - duplication of the 22p

Duplication Fragile X Syndrome caused by triplet repeats in a gene on the X chromosome  - the tip of the X chromosome appears to be attached only by a small thread  the number of repeats of the gene determines the severity of the disease

Translocation - a piece of one chromosome breaks off and attaches to another. Depending on where the break occurs, this may not be symptomatic. Inversion - a piece breaks off and reverses, then reattaches, this can be quite damaging because the underlying DNA code is now reversed and will not be readable.

Other Human Genetic Disorders Autosomal recessive Tay Sachs - lysosomal storage disorder Cystic Fibrosis - cell membrane disorder Sickle Cell Anemia (codominant)

Autosomal Dominant Disorders (Affected individuals are DD or Dd) Dwarfism Huntington’s Disease Polydactyly

Sex Linked Disorders Colorblindness Hemophilia

Why do our closest relatives have more chromosomes than we do? Fun Fact! The chimpanzee has 48 chromosomes, humans have 46. One hypothesis for this difference is that two chromosomes fused together in a distant ancestor.