Genetics and Heredity

• Mendel used pea plants to control mating, allowing him to be positive about the exact characteristics of each parent.
• The term P generation refers to the parent generation, F1 generation refers to the first generation of hybrids, and F2 generation refers to the second generation of hybrids.
• Gregor Mendel, an Austrian monk, discovered the fundamental laws of inheritance that built the foundation for modern day genetics in the second half of the 1800s.
• Klinefelter syndrome is the result of an extra chromosome in a male, producing XXY individuals.
• Monosomy X, called Turner syndrome, produces X0 females, who are sterile.
• Females with trisomy X (XXX) have no unusual physical features except being slightly taller than average.
• About one in every 1,000 males is born with an extra Y chromosome (XYY) and does not exhibit any defined syndrome.
• Mendel extensively studied the inheritance of various traits in pea plants and found that many of the traits followed predictable patterns.
• Mendel had no molecular mechanism to explain the results of his experiments; he had no knowledge of DNA or chromosomes.
• Instead, Mendel developed a model, with an included set of laws, to explain his experimental results.
• Mendel’s model includes four key concepts: alternative versions of genes account for variations in inherited characteristics, a homologous pair is a pair of genes that are identical except at a specific locus, the locus of the flower color gene is the location of the gene responsible for flower color, and the F1 generation is the first generation of hybrids.
• Each gene is a sequence of nucleotides at a specific place, or locus, along a particular chromosome.
• The DNA at that locus can vary slightly in its nucleotide sequence, which can affect the function of the encoded protein and thus the inherited characteristic of the organism.
• Each allele is a different DNA sequence variation.
• In chicken black feathers (C B ) and white feathers (C W ) are both dominant.
• Incomplete dominance in snapdragons means that red flower color (C R ) is incompletely dominant over white flower color (C W ).
• A testcross can be done to determine the genotype of an individual with the dominant phenotype, who could be either homozygous or heterozygous.
• A cross between heterozygotes for one characteristic is called a monohybrid cross.
• Multiple alleles are present in blood types.
• He then crossed two dihybrids, called a dihybrid cross.
• In hamsters, dark brown fur color (B) is incompletely dominant to white fur color (b).
• Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: when alleles are not completely dominant or recessive, when a gene has more than two alleles, when multiple genes influence a single phenotype, when the environment impacts the phenotype, when genes are not located on nuclear chromosomes.
• Mendel also followed multiple characteristics at the same time by creating dihybrids, plants that are heterozygous for two characteristics.
• In another species of hamsters, dark brown fur color (B) is codominant to white fur color (W).
• The heterozygous phenotype is somewhere between the phenotypes of the two homozygous phenotypes.
• In incomplete dominance, neither allele is completely dominant over the other in a heterozygous individual.
• The results of his dihybrid cross experiments showed that each pair of alleles separate independently during gamete formation.
• Complete dominance occurs when one allele completely masks the other allele in a heterozygous individual.
• If any offspring display the recessive phenotype, the mystery parent must be heterozygous.
• This is called the law of dominance.
• A testcross is when the mystery individual is crossed with a homozygous recessive individual.
• Mendel’s Model states that an egg or sperm only gets one of the two alleles that are present in the somatic cells of the organism.
• Mendel’s Model states that if the two alleles at a locus differ, then one, the dominant allele, determines the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance.
• Mendel’s Model states that for each characteristic, an organism inherits two copies (two alleles) of a gene, one from each parent.
• The organism's physical appearance is called its phenotype.
• Unlike homozygotes, heterozygotes are not true-breeding.
• Both the genotypic and phenotypic ratios can be determined for a cross.
• Today we know that each somatic cell in a diploid organism contains two sets of chromosomes, one set from each parent.
• An organism that has two different alleles for a gene is said to be heterozygous for the gene controlling that characteristic.
• A Punnett square is a diagram for predicting the results of a genetic cross between individuals of known genetic makeup.