Chapter 15: Chromosomal Basis of Inheritance

Chromosomal Basis of Inheritance

Overview: Locating Genes Along Chromosomes

The discovery that genes are located on chromosomes provided a physical basis for Mendel's laws of inheritance. Modern techniques allow scientists to visualize the location of specific genes on chromosomes using fluorescent dyes.

• Genes are the hereditary factors described by Mendel.

• Genes are physically located on chromosomes.

• The position of a gene on a chromosome is called its locus.

• Fluorescent tagging can highlight specific genes on isolated chromosomes.

The Chromosomal Basis of Mendel's Laws

Mendel's laws of segregation and independent assortment can be explained by the behavior of chromosomes during meiosis.

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation so that each gamete receives only one allele.

• Law of Independent Assortment: Genes located on different chromosomes assort independently during gamete formation.

Morgan's Experimental Evidence: Scientific Inquiry

Thomas Hunt Morgan used the fruit fly Drosophila melanogaster to provide the first solid evidence that specific genes are located on specific chromosomes.

• Fruit flies are ideal for genetic studies because they have only four pairs of chromosomes, short life cycles, and produce many offspring.

• Morgan discovered a mutant male fly with white eyes (instead of the wild-type red eyes), leading to the identification of sex-linked inheritance.

Wild-Type and Mutant Phenotypes

• Wild-type phenotype: The most common phenotype in a population (e.g., red eyes in fruit flies).

• Mutant phenotype: Traits that differ from the wild type due to mutations (e.g., white eyes in fruit flies).

Correlating Behavior of a Gene's Alleles with Chromosome Pairs

Morgan's crosses between white-eyed males and red-eyed females showed that the white-eye trait was inherited only by males in the F2 generation, indicating that the gene for eye color is located on the X chromosome.

• Supported the chromosome theory of inheritance.

Sex-Linked Genes and Patterns of Inheritance

Genes located on sex chromosomes (X or Y) exhibit unique inheritance patterns.

• The X chromosome carries many genes unrelated to sex determination.

• The Y chromosome is smaller and carries fewer genes, but includes the SRY gene that determines maleness in humans.

Sex Determination Systems

• Mammals: XX = female, XY = male. The SRY gene on the Y chromosome triggers male development.

• Grasshoppers: XX = female, XO = male.

• Birds: ZW = female, ZZ = male (female determines sex).

• Bees: Fertilized eggs (diploid) become females; unfertilized eggs (haploid) become males.

• Reptiles: Temperature during incubation determines sex in some species.

• Some fish: Can change sex during their lifetime.

Human Sex Chromosomes

• The human X chromosome has about 1098 protein-coding genes; most are unrelated to sex determination.

• The Y chromosome has about 78 genes and is crucial for male development.

• Humans can survive with an XO genotype (Turner syndrome), but not YO.

Sex-Linked Disorders in Humans

Many disorders are caused by recessive alleles on the X chromosome and are more common in males.

• Color blindness (mostly X-linked)

• Duchenne muscular dystrophy

• Hemophilia

Red-Green Color Blindness

• Caused by a recessive allele on the X chromosome.

• Affected individuals cannot distinguish between red and green.

Duchenne Muscular Dystrophy

• X-linked recessive disorder causing progressive muscle weakness.

• Primarily affects males; females are usually carriers.

Hemophilia A

• X-linked recessive disorder resulting in deficiency of a blood clotting factor.

• More common in males; females are often carriers without symptoms.

X Inactivation in Female Mammals

In female mammals, one of the two X chromosomes in each cell becomes inactivated during early development, forming a Barr body. This ensures dosage compensation between males and females.

• Inactivation is random in each cell, leading to mosaic expression of X-linked genes.

• Example: Tortoiseshell and calico cats, where different patches of fur express different X-linked color alleles.

Linked Genes and Genetic Recombination

Genes located close together on the same chromosome are called linked genes and tend to be inherited together. However, crossing over during meiosis can produce new combinations of alleles (genetic recombination).

• Linked genes do not assort independently.

• Genetic recombination occurs when crossing over exchanges segments between homologous chromosomes.

Mapping Genes Using Recombination Data

The frequency of recombination between two genes can be used to estimate their distance apart on a chromosome, creating a genetic map.

• One map unit (centimorgan, cM) corresponds to a 1% recombination frequency.

• The farther apart two genes are, the more likely a crossover will occur between them.

Summary Table: Sex Determination Systems

Organism	Sex Chromosome System	Sex Determination
Mammals	XX/XY	SRY gene on Y triggers male development
Grasshoppers	XX/XO	Presence or absence of X
Birds	ZW/ZZ	Female is ZW, male is ZZ
Bees	Haplodiploidy	Fertilized = female, unfertilized = male
Reptiles	Temperature-dependent	Incubation temperature determines sex

Key Equations

• Recombination frequency:

$\text{Recombination\ frequency} = \frac{\text{Number of recombinant offspring}}{\text{Total number of offspring}} \times 100\%$

Example: X-Linked Inheritance Punnett Square

For a cross between a carrier female (XNXc) and a normal male (XNY):

	XN	Y
XN	XNXN (normal daughter)	XNY (normal son)
Xc	XNXc (carrier daughter)	XcY (affected son)

Additional info:

• Male calico cats are rare and usually have an extra X chromosome (XXY), a condition known as Klinefelter syndrome.

• Genetic mapping and recombination frequencies are foundational for understanding gene linkage and inheritance patterns.