Genetics and Inheritance Notes

Heredity: Genetics and Inheritance Review

This is a review of genetics and inheritance. Consult a general biology text for more in-depth study if needed.

I. Genetics, Development, and Inheritance

A. Genes and Chromosomes

• DNA: The genetic material composed of individual nucleotides.

• Genes: Specific sequences of nucleotides on DNA that carry the code for the manufacture of specific proteins.

• Chromosomes: Condensed or coiled DNA along with proteins; DNA takes on this condensed configuration at the time of cell division. Humans have 23 pairs of chromosomes (homologous chromosomes) for a total of 46 chromosomes.

  • Autosomes: 22 pairs of chromosomes with general information.

  • Sex chromosomes: 1 pair which carry information about sex as well as some other traits; designated X and Y. Males are XY, and females are XX.

• Allele: The various forms (e.g., brown hair, black hair, blonde hair) of a given gene (e.g., hair color). Each person has two alleles for each trait – one from their mother and one from their father.

• Homozygous: Having two alleles that are the same – e.g., both alleles for brown hair.

• Heterozygous: Having two alleles that are different – e.g., an allele for red hair and an allele for brown hair.

• Dominant allele: An allele that is always expressed, even if the second allele is different (represented by an uppercase letter, e.g., "B" for brown).

• Recessive allele: An allele that is always masked or hidden if the dominant allele is present (represented by the lowercase letter of the dominant allele, e.g., "b" for red).

• Genotype: An organism’s specific genetic make-up; complete set of chromosomes present in all somatic (body) cells; each cell uses the portions of the DNA that it is programmed to use.

• Phenotype: Physical characteristics of an organism, such as blue eyes, brown hair, etc.

• Karyotype: Chromosomes arranged in pairs according to their size, shape, and general appearance; this is done to diagnose chromosomal abnormalities, such as a missing chromosome, an extra chromosome, etc.

B. Patterns of Inheritance

• Dominant/recessive inheritance: One allele is completely dominant (expressed), and one allele is completely recessive (not expressed).

  • Example: Cystic fibrosis – a lethal genetic disorder that codes for a faulty protein, leading to thickened mucus which clogs airways and increases the risk of respiratory infections.

• Incomplete dominance: Neither allele is completely dominant over the other; usually expressed as an intermediate trait between the two alleles.

  • Example: Red and white alleles in some flowers – if a flower has an allele for red and an allele for white, the color would be pink.

• Codominance: Neither allele is dominant over the other, and if both alleles are present, they are both expressed.

  • Example: In other flowers, red and white are codominant, and the expressed flower color is red and white striped.

• Polygenic inheritance: More than one gene contributes to the expression of a trait, such as in skin color, eye color, height, intelligence.

C. Predicting Inheritance

• The incidence of many traits, whose mode of inheritance is known, can be mathematically predicted as a probability.

• Punnett square: A simple technique for predicting the probability of inheriting a trait.

• Genetic counselors often prepare charts that trace a genetic trait through several generations in order to assess risk. Such a chart is known as a pedigree.

D. Sources of Individual Variation

• Genetic recombination

  • Independent assortment in meiosis

  • Crossing over: Occurs during prophase of meiosis I; chromosomes in synapsis (a very close binding of homologous chromosomes) may exchange pieces and reshuffle the genetic material (translocation).

  • Random fertilization

• Mutation

  • Spontaneous mutations: Happen for no known reason; usually caught and repaired by proofreading enzymes.

    • Deletions: A piece of a chromosome is lost.

    • Substitutions: One nucleotide is substituted for another.

    • Additions: Additional nucleotides are added.

  • Frame shifts: If nucleotides are added or deleted, then the sequence of nucleotides in the DNA triplets will be changed after the addition or deletion; this leads to misinformation after the mutation.

E. Sex-Linked Inheritance (or X-linked)

• Occurs when traits are carried on the X chromosome.

• These traits usually demonstrate dominant/recessive inheritance, so if the female has the recessive allele (the one that causes a disease, for example) on one X chromosome, she may have the dominant (healthy) allele on her other X chromosome – in this scenario, the female would be called a “carrier”; she carries the allele for the disease, but she does not express the disease; a woman can express the trait if she receives the recessive allele on both of her X chromosomes.

• The male, however, receives only one X chromosome, so if he carries the recessive allele (for disease), there will be no other X chromosome to mask it, and he will express the disease (remember, his other sex chromosome is the Y chromosome).

• Three common examples of sex-linked or X-linked traits are hemophilia, muscular dystrophy, and color-blindness.

F. The Human Genome Project

A joint public and private sector project to map the entire human genome, nucleotide by nucleotide. This project was begun in 1990 and completed in 2003 and will provide information for geneticists to work with for the remainder of the 21st century. This is the first step in learning where genes are on our chromosomes, which will eventually lead to ways to perhaps prevent, cure, or treat genetic disorders.

http://genomics.energy.gov