Mendelian Genetics

• Gregor Mendel used pea plants in his genetic experiments

Mendelian Genetics

• Mendel's experiments led to the discovery of the principles of inheritance
• If a plant has one dominant and one recessive allele for flower color, its phenotype will be the dominant color according to Mendel

Punnett Squares and Probability

• In a monohybrid cross (Aa x Aa), the probability of an offspring being homozygous recessive (aa) is 25%
• When crossing a heterozygous tall plant (Tt) with a short plant (tt), 50% of the offspring will be short
• In a dihybrid cross (AaBb x AaBb), the expected phenotypic ratio is 9:3:3:1

Sex-Linked Traits

• Males are more likely to inherit sex-linked traits because they only have one X chromosome, so a recessive allele on that X chromosome will be expressed
• A colorblind father passes his X chromosome to his daughters only
• The probability of a son having hemophilia if the mother is a carrier (XhX) and the father is normal (XY) is 50%

X-Chromosome Inactivation & Punnett Squares with Sex-Linked Traits

• X-chromosome inactivation is the random deactivation of one X chromosome in female cells to prevent overexpression of genes
• In a Punnett square, XY represents the male sex chromosome
• If a male with hemophilia (XhY) has children with a female who is not a carrier (XX), 100% of their daughters will be carriers

Incomplete Dominance

• Incomplete dominance occurs when a heterozygous phenotype is a blend of the two homozygous phenotypes
• Crossing a red flower (RR) with a white flower (WW) in incomplete dominance results in pink offspring (RW)
• If two pink flowers (RW x RW) are crossed, 50% of their offspring will be pink

Multiple Alleles & Blood Types

• Blood type is an example of a trait with multiple alleles
• The possible genotypes for Type A blood are IAIA or IAi
• The letter "I" in blood type notation stands for inheritance
• Type B blood has the B antigen
• A person with Type O blood has the genotype ii
• A mother with Type O blood (ii) and a father with Type AB blood (IAIB) could have a child with Type A or Type B blood

Punnett Squares for Blood Type

• If a person with blood type AB (IAIB) has a child with a person who has blood type O (ii), 50% of their offspring will have Type A blood
• A woman with Type A blood (IAi) and a man with Type B blood (IBi) could have a child with blood type A, B, AB, or O

Polygenic Traits & Epistatic Genes

• Height is a polygenic trait
• An epistatic gene can completely mask the effects of other genes
• Albinism is an example of an epistatic gene

Environmental Effects on Phenotype

• A plant growing taller in nutrient-rich soil demonstrates how the environment can affect phenotype
• A rabbit's fur changing color in winter due to temperature exemplifies environmental influence on phenotype

Gene Linkage & Independent Assortment

• Gene linkage refers to genes inherited together because they are close on the same chromosome
• Thomas Hunt Morgan's fruit fly experiments helped discover gene linkage
• Mendel stated that genes assort independently, while Morgan found that linked genes do not
• During meiosis, chromosomes assort independently meaning chromosomes separate randomly into gametes

Crossing Over & Genetic Distance

• Crossing over increases genetic diversity during meiosis
• Genes very close together on a chromosome are more likely to be inherited together
• The distance between genes affects whether they are inherited together, how often crossing over happens between them, and their recombination frequency

Wild Type & Mutant Traits

• In Morgan's fruit fly experiments, the "wild type" trait was the most common phenotype found in a population
• A mutant trait results from a genetic change

Linkage Maps & Crossover Frequency

• A linkage map shows the relative locations of genes on a chromosome
• Genes with a high crossover frequency are more likely to be far apart on the chromosome
• The unit of measurement for genetic distances on a linkage map is centimorgans
• If two genes are 10 map units apart, crossing over occurs between them 10% of the time

Pedigrees & Human Genetics

• A pedigree tracks how traits are inherited over generations
• In a pedigree, a shaded shape represents a person with the trait
• A half-shaded shape means the individual is a carrier
• In a pedigree, a horizontal line indicates marriage
• A vertical line connects parents to their offspring

Autosomal & Sex-Linked Traits

• A trait in every generation of a pedigree is likely autosomal dominant
• A single-gene trait is controlled by one gene
• Cystic fibrosis is an autosomal recessive disorder
• If two carrier parents (Aa x Aa) for an autosomal recessive disorder have children, 25% will be affected (aa)
• Cystic fibrosis is caused by a defective chloride ion transport protein
• Huntington's disease is inherited as autosomal dominant
• If a parent has Huntington's disease (Hh) and the other parent does not (hh), 50% of their children will inherit the disorder
• Males are more likely to inherit sex-linked disorders because they have only one X chromosome, so recessive traits are not masked
• A female carrier for a sex-linked disorder would have genotype XhX
• A man with hemophilia (XhY) has children with a woman who is NOT a carrier (XX), 0% of their sons will have hemophilia
• Hemophilia is an X-linked recessive disorder
• Males inherit their X chromosome from their mother

Karyotypes & Chromosomal Disorders

• A karyotype identifies an individual's chromosomes
• A karyotype provides information about chromosomal disorders, sex, and number of chromosomes
• Trisomy 21, also known as Down syndrome, is caused by an extra copy of chromosome 21
• Genetic disorders can be caused by mutations in DNA, environmental factors, and inherited gene mutations

Test Crosses & Punnett Squares

• A test cross determines the genotype of an individual with a dominant phenotype
• If an individual with genotype Aa is crossed with aa, 50% of offspring will show the recessive phenotype
• The chance that two heterozygous carriers of a recessive disorder will have a child with the disorder is 25%
• A child with type O blood must have parental genotypes ii x ii

Genetic Disorders & Inheritance Patterns

• A change in one nucleotide is associated with sickle cell anemia
• An autosomal recessive disorder is more likely to appear when both parents are carriers
• A carrier of an autosomal recessive disorder has one dominant and one recessive allele
• Cystic fibrosis is caused by a mutation in a single gene
• An individual with homozygous dominant alleles for an autosomal dominant disorder will have the disorder
• Cystic fibrosis is not a sex-linked trait