1. Chromosomes, Mapping, and the Meiosis–Inheritance Connection
Chapter 13

2. Learning outcomes What is the evidence for genes being on chromosomes?
Explain how mutations can cause disease.
What are the consequences of nondisjunction in humans?

3. Carl Correns – 1900 Walter Sutton – 1902
First suggests central role for chromosomes
One of papers announcing rediscovery of Mendel’s work
Walter Sutton – 1902
Chromosomal theory of inheritance
Based on observations that similar chromosomes paired with one another during meiosis

4. T.H. Morgan – 1910 Working with fruit fly, Drosophila melanogaster
Discovered a mutant male fly with white eyes instead of red
Crossed the mutant male to a normal red-eyed female
All F1 progeny red eyed = dominant trait

5. Morgan crossed F1 females x F1 males
F2 generation contained red and white- eyed flies
But all white-eyed flies were male
Testcross of a F1 female with a white-eyed male showed the viability of white-eyed females
Morgan concluded that the eye color gene resides on the X chromosome

8. Sex Chromosomes
Sex determination in humans is based on the presence of a Y chromosome
2 X chromosomes = female
Having a Y chromosome (XY) = male

9. © BioPhoto Associates/Photo Researchers, Inc.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
X chromosome
Y chromosome
2.8 µm
© BioPhoto Associates/Photo Researchers, Inc.
Humans have 46 total chromosomes
22 pairs are autosomes
1 pair of sex chromosomes
Y chromosome highly condensed
Recessive alleles on male’s X have no active counterpart on Y
“Default” for humans is female
Requires SRY gene on Y for “maleness” - masculinization of genitalia and secondary sex organs.

10. Hemophilia
Human genetic disorders show sex linkage when the relevant gene is on the X chromosome.
An example is hemophilia - Disease that affects a single protein in a cascade of proteins involved in the formation of blood clots
Form of hemophilia is caused by an X-linked recessive allele
Heterozygous females are asymptomatic carriers
Allele for hemophilia was introduced into a number of different European royal families by Queen Victoria of England

12. Dosage compensation Prevents doubling of sex-linked gene products.
Ensures an equal expression of genes from the sex chromosomes even though females have 2 X chromosomes and males have only 1
In each female cell, 1 X chromosome is randomly inactivated and is highly condensed into a Barr body

13. Females heterozygous for genes on the X chromosome are genetic mosaics
Allele for black coat is on one X, the allele for orange is on the other. Black appears when allele for orange is inactivated, and vice versa.

15. Calico cats are almost always female because the X chromosome determines the color of the cat and a female cat has two X chromosomes. A common male cat has one X and one Y chromosome. Since the Y chromosome does not have any color genes, there is no chance he could have both orange and non-orange together. One main exception to this is when, in rare cases, a male has XXY chromosomes, in which case the male could have tortoiseshell or calico markings. Most male calico or tortoiseshell cats are sterile due to the abnormality of carrying two X chromosomes.

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17. Genetic Mapping
Early geneticists realized that they could obtain information about the distance between genes on a chromosome
Based on genetic recombination (crossing over) between genes
If crossover occurs, parental alleles are recombined producing recombinant gametes
Genes close together on a single chromosome are said to be linked.
As physical distance on a chromosome increases, so does the probability of recombination (crossover). This allows genetic maps to be constructed based on the frequency of recombination.

20. Nondisjunction
Failure of homologues or sister chromatids to separate properly during meiosis
Aneuploidy – gain or loss of a chromosome
Monosomy – loss
Trisomy – gain
In all but a few cases, do not survive

21. Smallest autosomes can present as 3 copies and allow individual to survive
13, 15, 18 – severe defects, die within a few months
21 and 22 – can survive to adulthood
Down Syndrome – trisomy 21
May be a full, third 21st chromosome
May be a translocation of a part of chromosome 21
Mother’s age influences risk

22. Down syndrome Women 20-30 – 1 in 1,400 births.
By age 35 – 1 in 400
Over 45 – 1 in 35

24. Trisomy 13 (Patau Syndrome)
The incidence of Trisomy 13 is is approximately 1 out of 16,000 live births.
Trisomy 13 produces intellectual and physical handicaps, skull and facial abnormalities, and defects in all organ systems. It is also associated with a left lip, a large, triangular nose, and extra digits.
Eighty percent die in the first month, five to ten percent live past the first year.

25. Cri du Chat Syndrome
Cri du chat syndrome is due to a deletion of a portion of chromosome 5.
Cri du chat individuals are intellectually handicapped.
"Cri du chat" is French for "cry of the cat". The infants cry sounds like a cat.

27. Nondisjunction of sex chromosomes
Do not generally experience severe developmental abnormalities
Individuals have somewhat abnormal features, but often reach maturity and in some cases may be fertile

28. XXX – triple-X females – mild symptoms – may have learning disabilities.
XXY – males (Klinefelter syndrome) – males, hypogonadism, usually infertile
XO – females (Turner syndrome) – short height, broad chest, webbed neck, usually infertile
OY – nonviable zygotes
XYY – males (Jacob syndrome) – tall, thin males, may have social and learning disabilities.

30. Detection
Pedigree analysis used to determine the probability of genetic disorders in the offspring.
Amniocentesis collects fetal cells from the amniotic fluid for examination. Usually needle and fetus are observed with ultrasound.
Chorionic villi sampling collects cells from the placenta for examination

31. Amniocentesis
The fetus is surrounded by a layer of liquid called amniotic fluid. Amniocentesis - a sample of amniotic fluid is removed and cells that it contains are grown on a culture dish. Because these cells are of fetal origin, any chromosomal abnormalities present in the fetus will also be present in the cells.
Amniocentesis cannot be done until the 14th to 16th week of pregnancy. Cells must then be cultured on a laboratory culture dish for 2 weeks to obtain sufficient numbers of cells.

33. Chorionic villi
Chorionic villi sampling is a procedure in which a small amount of the placenta is removed.
It is normally done during the 10th to 12th week but it can be done as early as the 5th week of pregnancy. Karyotype analysis can be performed on these cells immediately after sampling. 

35. Two exceptions to Chromosomal Inheritance
Genomic Imprinting – the expression of a gene depends on whether it passes through the maternal or paternal germ line.
Ex. – Epigenetics
Epigenetic changes are inheritable but to not involve a change in the DNA

36. Mitochondria have their own genomes.
Passed to offspring in the egg and leads to maternal inheritance.