Genetic Inheritance Study Notes

Module 7: Genetic Inheritance

Genetic Characters are Inherited

  • Genetic characters (or traits) are inherited qualities received from parental DNA.
        - Physical Traits: These traits lead to specific shapes, structures, and colors (e.g., flower color, height).
        - Behavioral Traits: These traits affect neurochemical and endocrine differences.

Genetic Characters May Vary

  • Invariant Characters: Most genetic characters remain the same across all individuals of a species.
        - Example: All humans possess two eyes and the same bone structure.

  • Variable Characters: Some genetic characters can differ among individuals.
        - Example: Diverse eye colors or heights in humans.
        - Definitions: Different versions of a genetic character are called alleles.

Genetic Characters are Found on Genes

  • Genes: Segments of DNA located on chromosomes that encode genetic characteristics.
        - Example: The gene for flower color in pea plants, which has different alleles (purple or white).

Diploid Cells Have Two Alleles

  • Diploid Organisms: Have two copies of each chromosome, thus two copies of each gene.
        - Homozygous: An organism is homozygous if the alleles for a gene code for the same trait (e.g., PP or pp for flower color).
        - Heterozygous: An organism is heterozygous if the alleles code for different traits (e.g., Pp).

Dominant Alleles are Expressed

  • Dominant Alleles: Always expressed if present, masking other alleles.
        - Example: The purple allele (P) masks the white allele (p) for flower color.
        - Representation: Dominant alleles are typically represented with capital letters.

Recessive Alleles Can be Hidden

  • Recessive Alleles: Expressed only in the absence of dominant alleles.
        - Example: White flowers (pp) will only appear in the absence of a purple allele (P).
        - Representation: Recessive alleles are represented with lowercase letters.

Genotypes Dictate Phenotypes

  • Genotype: Specific combination of alleles for a particular gene (e.g., PP, Pp, or pp for flower color).

  • Phenotype: The physical appearance resulting from the genotype.
        - Example: Pea plants with genotypes PP or Pp (purple flowers) and genotype pp (white flowers).

Concept Check

  • Consider the three genotypes: AA (homozygous), Aa (heterozygous), aa (homozygous).
        - Questions:
            - Identify which are homozygous and heterozygous.
            - If A is dominant for tall plants and a for short, identify the dominant allele.
            - Determine which plants express tall or short phenotypes.

Alleles Will Separate in Meiosis

  • During sexual reproduction, alleles separate during gamete formation (meiosis).
        - Resulting gametes carry only one allele.
        - Each gamete has an equal probability of passing on either allele to offspring.

Punnett Squares Predict Offspring

  • Punnett Squares: Tools for predicting genetic probabilities and potential offspring genotypes and phenotypes.
        - Example: In Labrador retrievers, the B allele codes for black fur and the b allele for brown fur.

Creating a Punnett Square

  • Steps:
        1. Determine possible gametes from parental genotypes (e.g., for Pp: P and p).
        2. Draw a 2x2 grid to combine alleles from each parent.
            - Alleles from one parent are placed above columns; alleles from the other parent are placed beside rows.
        3. Combine alleles from the grid to find potential offspring genotypes and phenotypes.
            - Example: Parental genotypes of Pp lead to combinations like PP, Pp, pp.

Determining Offspring Ratios

  • Genotypic Ratios: Count occurrences of each possible genotype.
        - Example: For Pp x Pp, ratios are 1:2:1 for genotypes PP:Pp:pp, or as percentages: 1/4 or 25% PP, 2/4 or 50% Pp, 1/4 or 25% pp.

  • Phenotypic Ratios: Count occurrences based on the presence of the dominant allele.
        - Example: Ratio of purple to white flowers is 3:1, or 75% purple and 25% white.

Punnett Square Practice

  • Create Punnett squares for the following crosses:
        1. Pea plants for a gene coding seed color (Y for yellow, y for green): Yy x Yy.
        2. Pea plants for height (A for tall, a for short): AA x Aa.

Genetic Conditions are Often Recessive

  • Many genetic conditions are expressed only when both alleles are recessive.
        - Example: Deafness, sickle cell anemia, cystic fibrosis.
        - Heterozygous individuals carry the trait but do not express it.

Genetic Condition Punnett Square

  • Example: Hearing (D, dominant) vs. deaf (d, recessive).

  • Determine probabilities of offspring using a Punnett Square based on parental genotypes.

Pedigree Charts Determine Patterns

  • Pedigree Charts: Visual representations illustrating inheritance patterns of specific traits across generations.
        - Used to determine if traits are autosomal dominant or autosomal recessive.

Practice: Create a Pedigree Chart

  • Example Scenario: An unaffected mother and father have four children, one of whom is affected. Determine whether the trait is dominant or recessive and deduce genotypes using G and g.

Not All Traits Are That Simple

  • Many traits demonstrate more complex inheritance patterns:
        - Multiple alleles, incomplete dominance, co-dominance, etc.

Incomplete Dominance

  • Some genes express incomplete dominance, resulting in a blended phenotype (e.g., red and white flowers producing pink).
        - Crosses yield genotypic and phenotypic ratios of 1:2:1.

Incomplete Dominance Practice

  • Example: YY (yellow), BB (blue), YB (green). Calculate the likelihood of a yellow bird from a YB x YB cross.

Co-Dominance

  • Co-dominant alleles are both expressed; both are written as capital letters.
        - Example: Blood types with antigens on red blood cells.

Multiple Allele & Co-Dominance Example

  • Human blood groups involve three alleles: IA (blood type A), IB (blood type B), and i (blood type O).
        - IA and IB alleles are co-dominant and are dominant over i.

Blood Types are Phenotypes

  • Possible blood types include Type A (IAIA or IAi), Type B (IBIB or IBi), Type AB (IAIB), and Type O (ii).

Blood Type Punnett Square Practice

  • Determine probabilities for children’s blood types based on parents’ blood types.

Some Genes are Sex-Linked

  • Genes located on the X chromosome are sex-linked.
        - Males (XY) express X-linked traits regardless of dominance, as they have only one X chromosome.

Offspring Ratio is Always 50/50 XX/XY

  • Parents who are XX produce eggs with X alleles, while XY males produce sperm with either X or Y alleles, leading to a 1:1 ratio in offspring.

Colour Blindness is Sex-Linked

  • Colour blindness is a recessive X-linked trait.
        - Possible genotypes for XX: XNXN (not color-blind), XNXn (not color-blind), XnXn (color-blind).
        - Possible genotypes for XY: XNY (not color-blind), XnY (color-blind).

Sex-Linked Trait Practice

  • Determine the likelihood of a color-blind child from various parental genotypes (e.g., XNXn and XNY).

Genes Sort Independently

  • The inheritance of one gene does not generally affect the inheritance of another gene (Law of Independent Assortment).

Two Genes: Four Gametes

  • The inheritance of two genes results in four gametes, as alleles segregate independently.

Two Gene Example: Blood Types

  • Blood types also involve the Rh factor (Rh+ is dominant (D), Rh- is recessive (d)).

Determining Gametes with Two Genes

  • Analyze the genotypes of parents and identify their four possible gametes.

Determining Offspring with Two Genes

  • Create a 4x4 Punnett Square using gametes, and calculate the probabilities for specific phenotypes/genotypes.

Blood Type Inheritance Practice

  • Determine the possibility of certain blood types among children based on parent genotypes (e.g., A+ parents having O- child).

Genes Can Interact: Epistasis

  • Epistasis occurs when one gene affects the expression of another gene, despite the genes being inherited independently.

Epistasis Example: Labrador Retrievers

  • One gene codes for color (B = black, b = brown) and another for color expression (E = color, e = no color). If the dog is homozygous recessive for color expression (ee), it appears yellow regardless of color genes.

Epistasis Practice

  • Analyze genotypes (e.g., BbEe, bbee) for their phenotypes in Labradors, and determine possible offspring traits.

Traits Are Complex

  • Many human traits result from multiple genes and environmental factors, categorized as polygenic traits (e.g., eye color, skin color, height).

Vocabulary Check

  • Key Terms to Understand:
        - Gene: A unit of heredity, a segment of DNA coding for a specific trait.
        - Allele: Different versions of a gene.
        - Genotype: The genetic constitution of an organism.
        - Dominant Allele: An allele that expresses its trait in the presence of another allele.
        - Recessive Allele: An allele that is hidden in the presence of a dominant allele.
        - Homozygous Genotype: Contains two identical alleles for a trait.
        - Heterozygous Genotype: Contains two different alleles for a trait.

Practice Question

  • Given alleles D (dimples) and d (no dimples), identify genotypes and phenotypes of parents, and use a Punnett square to calculate the likelihood of their offspring having dimples.

Considerations

  • If both parents express a specific trait (e.g., black hair), is it possible for the offspring to not express that trait?

  • If neither parent expresses a trait (e.g., red hair), is it possible for the offspring to express that trait?