Patterns of Inheritance - Chapter 10

Patterns of Inheritance - Chapter 10

• Early attempts to understand inheritance had deep roots, with figures like Hippocrates and Aristotle struggling with this fundamental aspect of life.
• By the 19th century, it was established that offspring inherit traits from both parents, contradicting the prior belief that all traits come from the father. The mechanisms were, however, initially flawed.

Gregor Mendel

• Considered the father of classical genetics.
• Worked with pea plants in the 1860s, choosing them due to their rapid reproduction and readily observable traits (many varieties).
• Pea plants were easy to manipulate genetically.
• Established that parents pass heritable factors to their offspring.

Terms Coined by Mendel

• Hybrid: Offspring of two different genetic varieties (e.g., a purple-flowered plant and a white-flowered plant).
• Cross (or hybridization): Process of mating two different organisms.
• P generation: Parental generation.
• F1 generation: Offspring of the P generation.
• F2 generation: Offspring of the F1 generation.
• Monohybrid Cross: Cross of organisms differing in only one trait. This cross is often represented in a Punnett Square to show the resultant offspring probabilities.

More Terms

• Genotype: The actual genetic makeup (genes) of an organism.
• Phenotype: The observable characteristics (e.g. hair color, eye color, height) of an organism that are expressed from the organism's genotype .

Alleles

• Most organisms within a species have the same genes, but these genes can have variations, called alleles.
• Alleles are different forms of a gene, and these differences can lead to different versions of a particular trait (e.g. different eye colors).
• Homozygote/Homozygous: Means an individual has two identical alleles for a particular gene.
• Heterozygote/Heterozygous: Means an individual has two different alleles for a particular gene.

Dominance Versus Recessive

• When two genes have differing alleles and one is visible (fully expressed) the other hidden, the visible is called dominant; the hidden is called recessive.
• Dominant alleles are often symbolized with uppercase letters, and recessive alleles are often symbolized with lowercase letters.
• Homozygous dominant: AA; Homozygous recessive: aa; Heterozygous: Aa

Mendel's Seven Pea Plant Traits

• Traits observed in Mendel's classic experiment using peas. These include seed shape, seed color, flower color, pod shape, pod color, flower position, and stem height. These traits were tracked through generations to observe patterns of inheritance.

Punnett Square

• A device used to predict genotypes of offspring from a cross between two individuals.
• Used to determine potential genotype ratios.
• Can determine potential phenotype ratios.
• The parents' genotypes are used to generate potential offspring genotypes and their associated probabilities.

Hidden Alleles

• Organisms can carry recessive alleles that are not expressed if a dominant allele is present.
• These recessive alleles can still be passed down to offspring.

Mendel's Principle/Law of Segregation

• Pairs of genes separate (segregate) during meiosis and gamete formation to produce the "Gamete Set."
• During fertilization, these gametes fuse to form a new organism that returns to the diploid number once fertilization occurs.

Test Cross

• A method to determine the genotype of an organism with a dominant phenotype.
• Crosses the organism of unknown genotype with a recessive phenotype individual.

Mendel's Principle of Independent Assortment

• Different pairs of alleles segregate independently of each other during gamete formation.
• Linked genes (genes on the same chromosome) will often be inherited together, contradicting this principle; hence, they're not independent.

Conclusions

• Mendel's experiments using Pea Plants supported independent assortment of alleles.

Variations of Mendel's Principles

• Relationship between genotype and phenotype is rarely simple; multiple alleles often contribute to a single trait (e.g., eye color)
• Multiple genes often contribute to a single trait. Complex traits are often influenced by multiple genes.
• Eye color is a product of at least three genetic traits.

Incomplete Dominance

• When one gene is not dominant over the other, both genes express themselves which results in a "blending" of traits.

Co-dominance

• Neither allele is completely dominant, so both traits are expressed.
• ABO blood types are an example of co-dominance. The blood type is determined by the presence or absence of the A, B, or O carbohydrate on the surface of red blood cells. Different combinations result in different blood types.

Sickle Cell Anemia

• A genetic disorder resulting from a point mutation in the gene that codes for hemoglobin.
• Sickle cell trait results in a change in the shape and function of the red blood cells; in severe cases, this can lead to reduced oxygen carrying ability.

Pleiotropy

• One gene causes multiple traits.
• A single gene mutation changes an enzyme (phenylalanine hydroxylase) that results in the conversion of phenylalanine to tyrosine. In some cases, this can lead to build-up of dangerous toxic levels, affecting traits in several body systems. Phenylketonuria or PKU is an example of pleiotropy.

Regulatory Genes and Pleiotropy

• Regulatory genes control other genes to regulate processes (metabolism, development).
• Humans, chimpanzees, and bonobos are similar genetically, but differences in regulatory genes lead to different developmental outcomes in the organisms.

Epistasis or Polygenic Inheritance

• Two or more genes needed to complete a trait.
• The combined action of multiple genes can affect the outcome of a particular trait (e.g. eye color).
• Humans produce several pigments that influence eye color.
• Some traits are the result of pigments in birds; different genes encode for different pigments.

Linked Genes

• Genes located close together on the same chromosome tend to be inherited together.
• Linked genes do not follow Mendel's principle of independent assortment.

Inherited Disorders Caused by a Single Gene

• Autosomal Disorders: on non-sex chromosomes.
• Range from harmless (albinism) to deadly (Tay-Sachs).

Tay-Sachs

• Caused by a mutation in the HEXA gene on chromosome 15.
• This mutation results in a fatty acid (ganglioside) build-up that is toxic to the nervous system.
• The severity differs depending upon the type (infantile, juvenile, adult).

A genetic pedigree representing Tay-Sachs

• A family tree that illustrates the inheritance pattern of Tay-Sachs in multiple generations.
• Shows how the disorder is typically passed from parents to offspring.

Disorders Can Be Recessive or Dominant

• Most genetic disorders are recessive. A recessive disorder only manifests if an individual is homozygous recessive.
• Dominant genetic disorders are expressed in both homozygous and heterozygous forms.

Carriers

• People who carry a recessive allele for a disorder are called carriers, but typically do not show symptoms.
• Genetic testing helps detect fatal abnormalities and assist in informed parental decision-making.

Incest

• Closely related people share more similar genes and are more likely to carry the same recessive alleles.
• This increases the likelihood of offspring inheriting recessive disorders, particularly those resulting from shared recessive alleles.

Sex Chromosomes

• A pair of chromosomes that determines sex in most species.

Sex Identity

• The XX pair of chromosomes typically determines the female sex, whereas the XY pair is typically associated with the male sex.
• Other species have different chromosomal systems.

Sex-Linked Genes

• Genes found on the sex chromosomes.
• Some genes on the X chromosome affect traits not related to sex.

Mammals

• Sex-linked genes are located on the X chromosome.
• Genes on the Y chromosome are primarily associated with sex determination.
• Other species may have sex-linked genes on different chromosomes.

X-Linked Gene Expression

• Males inherit an X chromosome from their mothers; therefore all X-linked genes from their mothers are expressed.
• Females inherit one X chromosome from each parent, which means alleles from both parents are involved.

Recessive X-Linked Disorders

• Recessive disorders on the X chromosome manifest more frequently in males, where a single recessive allele is sufficient for the disease to be expressed.

Red-Green Color Blindness

• In males, color blindness occurs when a recessive allele on the X chromosome causes problems distinguishing shades of red from shades of green.
• Vision testing can help detect the level of colorblindness.

Hemophilia

• A disorder that prevents blood clotting.
• Can result from several different genetic mutations, often located on the X chromosome.
• Individuals with hemophilia may require supplemental clotting agents.

Explore the foundational concepts of inheritance as outlined in Chapter 10. This quiz delves into early theories, the pivotal contributions of Gregor Mendel, and the key terminology associated with genetics. Test your knowledge on how traits are passed from parents to offspring.