Genetics and Mendelian Inheritance Quiz

Patterns of Inheritance - Chapter 9

• Mendel's research on pea plants was based on large amounts of quantitative data and highly controlled experiments.
• Pea plants were a suitable species for study due to ease of growth, easily distinguishable traits, and the ability to control pollination.
• Gregor Mendel is considered the "father of modern genetics."
• His research demonstrated the inheritance of discrete traits, not blended traits as previously believed.

History of Genetics

• Heredity is the study of traits passed from parents to offspring.
• Genetics is the branch of biology that studies heredity.
• Early attempts to explain heredity date back to ancient Greece.
• Early theories included pangenesis (life contained within gemmules), and the blending hypothesis (offspring are a blend of parents).
• Mendel's work disproved the blending hypothesis, and established the field of modern genetics.

Gregor Mendel and His Pea Plants (Traits)

• Flower color: Purple is dominant; White is recessive
• Flower position: Axial is dominant; Terminal is recessive
• Seed color: Yellow is dominant; Green is recessive
• Seed shape: Round is dominant; Wrinkled is recessive
• Pod shape: Inflated is dominant; Constricted is recessive
• Pod color: Green is dominant; Yellow is recessive
• Stem length: Tall is dominant; Dwarf is recessive

Mendel's Classic Crosses

• Mendel always started with true-breeding (homozygous) varieties.
• The first generation is the P generation (parental).
• The second (F1) generation is the offspring of the P generation.
• The third (F2) generation is the offspring of the F1 generation.
• Mendel's monohybrid crosses involved studying one trait at a time.
• A cross between a purebred purple and a purebred white flower resulted in all purple offspring in the F1 generation.
• The F2 generation from the F1 cross showed a 3:1 ratio of purple to white flowers.
• This consistency in results was repeatable for all seven traits.

Rules of Inheritance

• Each trait is controlled by a gene having two alleles.
• For each trait, an organism inherits two alleles—one from each parent—which can be the same (homozygous) or different (heterozygous).
• One allele is dominant, always expressed if present. The other allele is recessive and only expressed in the absence of the dominant allele.
• During reproduction, each parent randomly passes one of the two alleles to offspring (Law of Segregation).

Punnett Square

• A mathematical model used to predict the possible outcomes of a cross between two parents.
• Phenotype is the physical expression of a trait (e.g., purple or white flowers).
• Genotype is the actual genetic makeup (e.g., PP, Pp, or pp).

Meiosis and Heredity

• Meiosis is the mechanism behind the law of segregation.
• Homologous chromosomes have the same gene loci but can have alternative alleles.
• During meiosis, homologous chromosomes split, and only one chromosome (with one allele) goes into each individual gamete.

Dihybrid Crosses

• Mating parental varieties that differ in two traits.
• Purebreds (P generation) are crossed.
• The resulting F1 generation is heterozygous dominant for both traits.
• The F2 generation exhibits a 9:3:3:1 phenotypic ratio.
• Alleles for separate traits are inherited independently (Law of Independent Assortment).
• This only works for genes on separate chromosomes or distant on the same chromosome.

Test Cross

• Used to determine the genotype of an organism with a dominant phenotype.
• The unknown organism is crossed with a known homozygous recessive organism.
• The phenotypes of the offspring are studied, and this is used to deduce the unknown parent's genotype.

Rules of Probability

• The likelihood of two or more independent events occurring simultaneously is calculated by multiplying their individual probabilities (rule of multiplication).
• The probability of an event happening in two or more different ways is calculated by adding their individual probabilities (rule of addition).

Pedigree

• Used to study the inheritance of traits or disorders through generations of a family.
• Can help identify carriers of recessive disorders (heterozygotes).

Simple Inheritance in Humans

• Traits governed by Mendel's rules ("simple inheritance") follow patterns with only two alleles (one dominant and one recessive).

Some Autosomal Disorders in Humans

• Table shows various recessive and dominant disorders with their symptoms, incidence, and comments.

Genetic Screening

• Several tests are available to identify potential genetic abnormalities in unborn children.
• Amniocentesis (14–16 weeks) and Chorionic Villus Sampling (8-12 weeks) are used to collect and analyze fetal cells.
• Blood tests and ultrasounds are also used.
• Newborn screening (e.g., PKU) can detect genetic conditions early.

Complex Patterns of Inheritance

• There are traits that do not follow simple Mendelian patterns (incomplete dominance, codominance, multiple alleles, pleiotropy, polygenic inheritance, genetic linkage, sex linkage).

Incomplete Dominance

• The heterozygote has an intermediate phenotype between the two homozygous traits.
• An example discussed was hypercholesterolemia.

Codominance

• Both alleles are equally dominant and contribute equally to the phenotype of a heterozygous individual.
• An example discussed was the different coat colors in cattle and chickens.

Multiple Alleles

• More than two alleles determine a trait.
• An example of this is ABO blood type, important in transfusions.

Pleiotropy

• A single gene affects multiple characteristics/traits.
• Examples presented include cystic fibrosis, phenylketonuria (PKU), and sickle cell disease.
• Sickle cell disease demonstrates heterozygote advantage, where the heterozygous condition provides resistance to malaria.

Polygenic Inheritance

• Several genes influence a single trait and influence variation along a continuum (e.g., height, skin color) rather than a simple phenotypic distinction.

Genes and the Environment

• Environmental factors can influence the expression of some genetic traits.
• Some traits are strictly genetic (e.g., blood type), others are influenced strongly by the environment, and many traits fall somewhere in between.

The Chromosomal Theory of Inheritance

• Chromosome behavior in mitosis and meiosis explains Mendel's results.
• Genes occupy specific loci on chromosomes, and the chromosomes undergo segregation and independent assortment.

Linked Genes

• Genes that are located close together on a chromosome tend to be inherited together.
• Crossing over can rearrange linked genes.
• The results of crosses involving linked genes don't always conform to typical Mendelian ratios (9:3:3:1).

Crossing Over

• Crossing over rearranges linked genes, potentially leading to recombinant offspring that don't exhibit parental phenotypes.
• The frequency of recombination can be used to map the relative location of linked genes on a chromosome.

Sex Chromosomes

• The 23rd pair in humans consists of sex chromosomes (XX in females and XY in males).
• In some species, the sex of the offspring is determined exclusively by the sperm.
• In other species, the female's sex chromosomes determine gender. Some species lack sex chromosomes altogether.

Sex-Linked Traits

• Traits carried on sex chromosomes (primarily the X chromosome).
• Males are more likely to express recessive sex-linked traits because males only have one X chromosome.
• Examples include red-green color blindness and hemophilia.