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Single-Gene Inheritance

4.1 Following the Inheritance of One Gene-Segregation

Mendel the Man
1. Born in what is now the Czeck Republic.
2. Grew up in an agricultural environment.
3. Became a priest-teacher at a local monastery.
4. Took courses in botany, physics, and mathematics at the University of Vienna.
5. Carried out "hybridization" experiments with the common garden pea.
6. Mendel's hypotheses became the laws of inheritance in modern genetics.
7. Mendel's work re-discovered by Correns, DeVries, and Tschermak in 1900. Period of modern "Mendelian" genetics begins.

Mendel's Experiments
1. Choose to work with the common garden pea, capable of self-fertilization. True breeding parental varieties easy to establish.
2. Monohybrid cross: Hybrid cross involving a single trait with two expressions (i.e. plant height; short vs. tall).
3. Results of monohybrid cross demonstrates dominant vs. recessive behavior and the law of segregation (i.e. tall pea plants could produce short offspring).
4. Mendel's "elementen" are what we call genes (or alleles).

Terms and Tools to Follow Segregating Genes
1. A homozygous individual possesses two identical alleles (i.e. AA or aa). A heterozygous individual possesses two different alleles (i.e. Aa).
2. Phenotype is the outward expression of a trait (i.e. tall vs. short; blue eyes vs. brown eyes).
3. Genotype is the actual genetic makeup of the individual (i.e. AA, Aa, aa).
4. Wild type refers to the most common form. A mutant is a variant that has under gone a mutation (change in the DNA).
5. The physical nature of meiosis (chromosome behavior) explains the law of segregation. The law of segregation states that inherited "characters" (alleles) separate during meiosis, so that each offspring receives one copy of each allele from each parent.
6. Punnett squares are a convenient method for diagramming a genetic cross. Inspection of the square gives you the genotypic and phenotypic results and ratios.
7. The genotypic ratio for a monohybrid cross is 1:2:1, and the phenotypic ratio is 3:1.
8. A test cross reveals the presence recessive genes in an individual with an unknown genotype by crossing them with an individual homozygous recessive for the genes in question.

4.2 Single-gene Inheritance in Humans

Modes of Inheritance
1. Modes of inheritance are the rules explaining the common patterns of inheritance.
2. A Mendelian trait is caused by a single gene.
3. Traits can be dominant or recessive and recur in a predictable pattern in subsequent generations.
4. Autosomal Dominant Inheritance: Autosomal dominant traits do not generally skip generations and can affect both sexes.
5. Autosomal Recessive Inheritance: Autosomal recessive traits can skip generations and can affect both sexes. Blood relatives that have children together have a much higher risk of having a child with a rare recessive disorder.
6. Punnett squares apply Mendel's first law to predict recurrence risks for inherited disorders or traits.
7. A Mendelian trait applies anew to each child.

On the Meaning of Dominance and Recessiveness
1. At the biochemical level, recessive disorders often result from alleles that cause the loss of function or production of a normal protein.
2. Dominant disorders can result from production of an abnormal protein that interferes with the function of a normal protein or result from a gain of function.

4.3 Following the Inheritance of Two Genes-Independent Assortment
1. Mendel's law of independent assortment considers genes transmitted on different chromosomes.
2. The phenotypic ratio of 9:3:3:1 of a dihybrid cross indicates that a gene on one chromosome does not influence transmission of a gene on a different chromosome.
3. In meiosis, random assortment of maternally and paternally derived chromosomes results in gametes that have different combinations of genes.
4. Punnett squares and probability are used to predict recurrence of more than one trait.

4.4 Pedigree Analysis

Pedigrees Then and Now
1. Pedigree charts depict family relationships and transmission of inherited traits.
2. Squares represent males and circles represent females.
3. Horizontal lines indicate parents, vertical lines show generations, and elevated horizontal lines depict siblings.
4. Symbols for heterozygotes are half-shaded, and for individuals with a particular phenotype, completely shaded.

Pedigrees Display Mendel's Laws
1. Pedigrees can reveal mode of inheritance, and can include molecular information, carrier status, and input from other genes and the environment.
2. Interpretation of pedigrees can be complicated by lack of information, adoption, children born out of wedlock, assisted reproductive technologies (i.e. artificial insemination), lack of penetrance.

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