12.1 Chromosomal Inheritance
Thomas Hunt Morgan, along with his colleagues, formulated Sutton's Chromosome Theory of Inheritance in the early 20th century. This theory proposed that chromosomes are the carriers of genetic material and that their behavior during cell division (specifically, the process of meiosis) is responsible for the transmission of traits from parents to offspring.
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Chromosomes carry genetic information: Sutton proposed that chromosomes, which were observed during cell division, were the carriers of hereditary information.
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The behavior of chromosomes during meiosis: Sutton observed the process of meiosis, where chromosomes pair up, exchange genetic material (crossing over), and segregate into gametes (sperm and egg cells). This process ensures that each gamete receives a set of chromosomes containing a mixture of genetic information from both parents.
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Chromosomes determine traits: According to Sutton's theory, specific genes reside on chromosomes and their arrangement and behavior during meiosis dictate the inheritance of traits from parents to offspring.
Autosomal Inheritance
Most genetic problems we've encountered so far have been about making predictions about inheritance that looked at genes ONLY on autosomes.
Ex: Aa x Aa = 3:1 ratio .......this only works if the alleles are on autosomes
AaBb x AaBb = 9:3:3:1 ratio* ....again this only works if they alleles are on separate autsomes
*the ratio illustrates Mendel's principle of Independent Assortment
Chromosomal Inheritance
Each species can have a different number of chromosomes, it is specific to the species and is one factor that can prevent hybridization between species (chromosomes must match during fertilization)
Humans have
22 pairs of autosomes, 1 pair of sex chromosomes
Fruit flies have 3 pairs of autosomes, 1 pair of sex chromosomes
Shown below: fruit fly and human karyotypes
Sex Linked Genes
Genes located on the X chromosome are inherited with that X. When doing crosses you must include the sex chromosomes in your cross. Use superscript letters for the allele.
Example: In fruit flies, eye color is a sex linked trait. Red is dominant to white.
Red eyed male x white eyed female : XRY x XrXr
All the males will be white-eyed, all the females will be red-eyed.
Human X-Linked Disorders
Colorblindness – unable to distinguish certain colors, the most common for is red-green colorblindness that affects primarily males
Muscular Dystrophy
- characterized by the wasting away of muscles
- life expectancy: ~20 yrs
- females can be carriers for the disease, where they pass the disease to their sons only
Hemophilia
- also known as "bleeder's disease", blood does not clot properly
- disease was present in the royal family, starting with Queen Victoria
Fragile X Syndrome
- caused by triplet repeats in a gene on the X chromosome
- causes mental retardation
- named because the X chromosome had an odd appearance - the tip seemed to be attached only by a small thread
- the number of repeats of the gene determines the severity of the disease
