Inheritance: Genotypes, Phenotypes, and Modes of Inheritance

Basic Terminology

• TRAIT: A distinct, inherited characteristic or feature of an organism (e.g., specific eye color, hair texture, height).

  • Traits are passed down from parents to offspring and contribute to an individual's unique characteristics.

• PHENOTYPE: The observable characteristics or traits of an organism, resulting from the interaction of its genotype with the environment; encompasses appearance, behavior, and physiological properties.

  • Includes physical appearance, biochemical and physiological properties, and behavior.

  • Examples: eye color, blood type, disease susceptibility.

• GENES: The fundamental units of heredity; segments of DNA that contain the instructions for building specific proteins or performing specific functions in the cell.

  • Genes provide the blueprint for an organism's traits.

• ALLELES: Different versions of a gene, each coding for a specific variation of a trait (e.g., blue, green, or brown eyes).

  • Individuals inherit two alleles for each gene, one from each parent.

• GENOTYPE: The genetic makeup of an organism; the specific combination of alleles it possesses for a particular gene or set of genes.

  • Determines the potential range of phenotypes that an organism can exhibit.

• Offspring traits may show patterns reflecting alleles donated from parents.

• Dominant alleles mask recessive alleles.

  • When a dominant allele is paired with a recessive allele in a heterozygote, the dominant allele's trait is expressed.

  • Recessive traits are only expressed when an individual is homozygous for the recessive allele.

• Example: A foal inheriting the “paint” trait (spots) from its father.

Patterns of Inheritance: Simple Dominance

• Organisms can be heterozygous or homozygous for a trait.

  • Heterozygous: Alleles for a trait are different.

  • Possessing two different alleles for a particular gene.

  • Example: Having one allele for brown eyes and one allele for blue eyes.

  • Homozygous: Alleles for a trait are the same.

  • Possessing two identical alleles for a particular gene.

  • Example: Having two alleles for brown eyes or two alleles for blue eyes.

• Example: One parent with blue eyes, one with brown eyes, offspring with brown eyes.

  • Brown allele is dominant to blue allele.

  • In this case, the brown eye allele masks the expression of the blue eye allele.

  • Offspring is heterozygous (brown allele and blue allele).

Punnett Square

• A Punnett Square is a diagram used to predict the probability of offspring genotypes and phenotypes based on parental crosses.

• Dominant alleles are written with an uppercase letter; recessive alleles with a lowercase letter.

• Example: Brown eye allele is "B", blue eye allele is "b".

• Homozygous blue-eyed parent genotype: bb

• Heterozygous brown-eyed parent genotype: Bb

Punnett Square Examples

• Allele for brown eyes (B) is dominant to the allele for blue eyes (b).

• Blue eye allele (b) is recessive and masked by the dominant (B) allele for brown eyes.

• Punnett Square A: One parent is heterozygous and the other is homozygous. 100% chance the offspring will have brown eyes.

• Punnett Square B: Both parents have blue eyes, both are homozygous. 100% chance the offspring will have blue eyes.

• Punnett Square C: One parent is heterozygous and the other is homozygous. 50% chance the offspring will have brown eyes and a 50% chance the offspring will have blue eyes.

• Punnett Square D: Both parents are heterozygous. 75% chance the offspring will have brown eyes and a 25% chance the offspring will have blue eyes.

Practice Questions: Sickle Cell Disease

• Sickle Cell Disease is an autosomal recessive inherited disease.

  • Requires two copies (homozygous) of the recessive sickle cell allele to show the disease trait.

• If a heterozygous mother without sickle cell disease and a father with sickle cell disease have a child:

  • Mother's genotype: Ss

  • Father's genotype: ss

  • Possible child genotypes: Ss or ss

  • Chance of child having sickle cell disease: 50%

Dihybrid Cross

• Uses a Punnett Square to predict offspring genotypes with more than one trait.

• Example:

  • Hair color: A for black hair, a for blonde hair.

  • Eye color: B for brown eyes, b for blue eyes.

• Use a dihybrid cross to predict likelihood of offspring’s hair and eye color.

Determining Possible Allele Combinations

• Genotype AaBb: Black hair, brown eyes, heterozygous for both traits.

Dihybrid Cross Example

• Two people with black hair and brown eyes, heterozygous for both traits (AaBb).

  • 9/16 chance their child will have black hair and brown eyes.

  • 3/16 chance their child will have black hair and blue eyes.

  • 3/16 chance their child will have blonde hair and brown eyes.

  • 1/16 chance their child will have blonde hair and blue eyes.

Patterns of Inheritance: Codominance & Incomplete Dominance

• Sometimes alleles do not follow a simple pattern of being dominant or recessive.

• Other patterns include incomplete dominance and codominance.

• Example: Human blood type, which does not follow simple dominance.

• Human blood types: Type A, Type B, Type AB, and Type O.

• Blood type alleles: I^A, I^B, i^O

  • I is dominant to i, but I^A and I^B are codominant.

Blood Type Genotypes

• A: I^Ai^O or I^AI^A

• B: I^Bi^O or I^BI^B

• AB: I^AI^B

• O: i^Oi^O

Practice Questions

• Mother has Type O blood, father has Type AB blood.

  • Mother's genotype: i^Oi^O

  • Father's genotype: I^AI^B

  • Possible offspring genotypes: I^Ai^O, I^Bi^O

  • Chance of having Type A blood: 50%.

• Mother has Type O blood, father is heterozygous for Type A blood.

  • Mother's genotype: i^Oi^O

  • Father's genotype: I^Ai^O

  • Possible offspring genotypes: I^Ai^O, i^Oi^O

  • Chance of having Type O blood: 50%

Sex-Linked Traits

• Traits carried on sex chromosomes (X or Y chromosome).

• Females: XX (homozygous), inherit one X from each parent.

• Males: XY (heterozygous), inherit X from mother, Y from father.

• More X-linked than Y-linked traits.

• Males are more susceptible to sex-linked conditions because they have only one X chromosome.

• Females are often carriers.

• Recessive sex-linked traits are more common than dominant traits.

• Ova cannot carry a Y chromosome.

X-Linked Hemophilia

• Some types of hemophilia are X-linked traits.

• Mothers can be carriers if only one X chromosome carries the hemophilia allele.

• Heterozygous females are carriers, do not display the trait, but can pass it to their children.

• Sons will show the disease because the recessive allele on their X chromosome is expressed.

Predicting Sex-Linked Traits

• Write X and/or Y chromosomes with the trait attached in Punnett Squares.

• Example: H for no hemophilia, h for hemophilia (recessive).

• Father without hemophilia (X^HY), mother is a carrier (X^HX^h).

  • 0% chance of affected daughter (X^hX^h)

  • 50% chance of affected son (X^hY)

Practice Questions: Colorblindness

• Colorblindness (b) is a recessive sex-linked trait on the X chromosome.

• Carrier mother (X^BX^b), colorblind father (X^bY).

  • Possible offspring genotypes: X^BX^b, X^bX^b, X^BY, X^bY

  • 25% chance of colorblind son (X^bY).

  • 25% chance of colorblind daughter (X^bX^b).

  • 50% overall chance of a child with colorblindness.

Pedigree Analysis

• Pedigrees track traits through generations.

• Dominant traits tend to show up in every generation.

• Sex-linked traits tend to show up more in males.
  
  *Example: Hemophilia among Queen Victoria's relatives. (Many affected male relatives).

• Pedigree charts use shaded/unshaded squares (males) and circles (females) to represent affected/unaffected individuals.

Practice Questions: Sickle Cell Disease

• Grandmother (I2) with sickle cell disease has three children.

• Two children (II3 son, II5 daughter) have sickle cell disease.

• Two male grandkids (III4, III5) and one female grandkid (III3) have sickle cell disease.

• S = no sickle cell, s = sickle cell (autosomal recessive).

  • Grandmother (I2): ss, Grandfather (I1): Ss

  • II children: II2: Ss,II3: ss ,II5: ss

  • Grandchildren (III1-6): Grandchildren (III1-6): III1:Ss, III2:Ss ,III3:ss ,III4:ss, III5:ss,III6:Ss