Chromosome Mapping & Meiosis-Genetics Chapter 13

Chromosome Mapping & Meiosis-Inheritance Connection

• Chapter 13 covers the relationship between chromosomes, mapping, and inheritance during meiosis.

X-Linked Genes in Drosophila

• T.H. Morgan's experiments with fruit flies (Drosophila melanogaster) demonstrated X-linked inheritance.
• Crossing a mutant white-eyed male with a normal red-eyed female produced all red-eyed F1 offspring.
• Crossing F1 females with F1 males, the F2 generation had both red and white-eyed flies, but all white-eyed flies were male.
• A testcross confirmed that the eye color gene resides on the X chromosome.

Sex Determination

• Sex determination varies across species.
  • Humans: presence of a Y chromosome determines maleness (XY), absence of Y results in femaleness (XX).
  • Drosophila: number of X chromosomes determines sex. 2X's = female, 1X = male
  • Birds and some other species use different chromosome combinations
  • Grasshoppers and Honeybees have other methods

SRY Gene and Male Development

• In humans, the SRY gene on the Y chromosome encodes the testis-determining factor (TDF), which initiates male development.
• Swyer syndrome is associated with a nonfunctional SRY gene, leading to a male genotype (XY) but a female phenotype.

Androgen Insensitivity Syndrome

• Androgen insensitivity syndrome results from mutations in the androgen receptor gene on the X chromosome. This leads to a male genotype(XY) but a female phenotype.

Sex-Linked Disorders

• X-linked disorders, like hemophilia, are more prevalent in males because they only have one X chromosome.
• Hemophilia (a blood clotting disorder) is caused by an X-linked recessive allele.
• X-linked disorders can appear in both males and females, but males are more often affected.

Mating Results: Hemophilia

• A mating between a normal (non-carrier) female and a hemophiliac male results in carriers but no affected daughters, and a chance of the sons being affected with hemophilia.

White-Eyed Fruit Fly Cross

• A cross between a homozygous red-eyed female and a white-eyed male will result in all male offspring having white eyes.

Dosage Compensation and Barr Bodies

• In females, one X chromosome is inactivated in each cell to prevent a doubling of gene products compared to males (one X).
• The inactivated X chromosome condenses into a Barr body.

Calico Cats

• Calico cats are females whose heterozygous alleles are responsible for the patches of different colors. The difference in color are in relation to the inactivation of one of the X chromosomes's alleles.

Mitochondrial DNA Inheritance

• Mitochondrial DNA is only inherited from the mother.

Nondisjunction and Aneuploidy

• Nondisjunction is the failure of homologous chromosomes to separate properly during meiosis.
• Aneuploidy is the result of nondisjunction; it refers to having an abnormal number of chromosomes (missing one less, one extra).
• Conditions like Klinefelter syndrome (XXY) and Turner syndrome (XO) are examples of sex chromosome abnormalities caused by nondisjunction.

Autosomal Chromosome Abnormalities

• Edwards syndrome (trisomy 18) and Down syndrome (trisomy 21) are examples of autosomal trisomies, which often result in significant health problems and may cause death. Both are related to the mother's age at time of pregnancy

Genetic Disorder Testing

• Amniocentesis and chorionic villus sampling (CVS) are prenatal testing procedures for diagnosing genetic disorders. These methods obtain fetal cells from amniotic fluid or chorionic villi, respectively, facilitating karyotyping and biochemical tests.. Both methods may potentially have risks of causing damage/miscarriage to the fetus

Linked Genes and Recombination

• Linked genes are on the same chromosome and tend to be inherited together.
• Crossing over during meiosis can result in recombination, producing gametes with new combinations of alleles.
• Recombination frequency is used to estimate the distance between genes on a chromosome, map units or centimorgans (cM) are used to measure this.

Gene Distance Determination

• The frequency of recombination between genes provides an estimation of the relative distance between them on a chromosome, or how far apart they are on a chromosome. Genes far apart are more likely to be separated during meiosis