Inheritance Notes

Inheritance

Definitions

• Inheritance: The process by which genetic information is passed from parents to offspring through DNA in gametes (sperm and egg cells).
• DNA (Deoxyribonucleic Acid): A molecule that carries genetic instructions for the development, functioning, and reproduction of all living organisms.
• Chromosome: A thread-like structure made of DNA and proteins, found in the nucleus, carrying genes.
• Gene: A segment of DNA that codes for a specific protein, determining a particular trait.
• Allele: A different version of the same gene, which can result in variations of a trait.

Inheritance - Chromosomes and Alleles

• Alleles are represented on chromosomes.
• Individuals have two alleles for each gene, one on each chromosome of a homologous pair.
• Possible allele combinations:
  • Homozygous Dominant: AA
  • Heterozygous: Aa
  • Homozygous Recessive: aa

Mendelian Genetics (Dominant and Recessive)

• Dominant Allele: An allele that is always expressed in the phenotype, even if only one copy is present (e.g., B in Bb or BB).
• Recessive Allele: An allele that is only expressed in the phenotype if two copies are present (e.g., b in bb).
• Example:
  • Brown eyes (B) are dominant over blue eyes (b).
  • A person with BB or Bb will have brown eyes, while only bb results in blue eyes.

Dominant and Recessive Alleles Explained

• Dominant Allele:
  • Expressed in the phenotype even if only one copy is present.
  • Higher chance of expression.
  • Masks the effect of a recessive allele.
  • Example: Brown eyes (B) are dominant over blue eyes (b), so BB or Bb = Brown eyes.
• Recessive Allele:
  • Only expressed in the phenotype if two copies are present.
  • Lower chance of expression.
  • Masked by a dominant allele.
  • Example: Blue eyes (b) are recessive, so only bb = Blue eyes.

Definitions

• Homozygous Dominant: An individual with two identical dominant alleles for a gene (e.g., BB for brown eyes).
• Heterozygous: An individual with one dominant and one recessive allele for a gene (e.g., Bb, where the dominant allele is expressed).
• Homozygous Recessive: An individual with two identical recessive alleles for a gene (e.g., bb for blue eyes).

Practice 1

• Part 1: Cat color inheritance
  • White (W) is dominant, Black (B) is recessive.
  • Two white cats crossed and produced a black offspring.
  • Explanation: Both white cats must be heterozygous (Wb), carrying the recessive black allele. When both parents contribute the recessive allele (b), the offspring is bb and expresses the black phenotype.
• Part 2: Flower color inheritance
  • Cross between a heterozygous white flower and a pure white flower.

Co-dominance

• Codominance occurs when both alleles in a heterozygous individual are fully expressed, resulting in a phenotype that shows both traits without blending.
• Explanation:
  • Unlike dominant and recessive inheritance, where one allele masks the other, in codominance, both alleles contribute equally to the phenotype.
  • This means that a heterozygous individual will have both traits visible rather than a mix of them.

Practice - Codominance in Cattle Coat Color

• In cattle, coat color follows a codominant inheritance pattern:
  • RR: Red coat
  • WW: White coat
  • RW: Roan coat (both red and white hairs are present)
• If a roan cow (RW) is crossed with another roan cow (RW), what are the possible genotypes and phenotypes of the offspring?

Answer to Codominance Practice Problem

• Punnett Square:

• Genotypes and Phenotypes:
  • RR (Red coat) → 1/4 (25%)
  • RW (Roan coat) → 2/4 (50%)
  • WW (White coat) → 1/4 (25%)
• Offspring Genotype and Phenotype Ratio: 1:2:1

Blood Type

• Blood type (ABO group) is inherited through multiple alleles and follows both dominance and codominance.
• Blood Group Alleles:
  • I^A (A allele) – Dominant
  • I^B (B allele) – Dominant
  • i (O allele) – Recessive
• Genotype to Blood Type (Phenotype) Mapping:
  • I^AI^A or I^Ai: A
  • I^BI^B or I^Bi: B
  • I^AI^B: AB (Codominant – both A & B expressed)
  • ii: O

Practice 1 - Blood Type Inheritance

• A heterozygous type A parent (I^Ai) and a type O parent (ii) have children.
• What are the possible blood types of their offspring?

X-Linked Inheritance

• What is X-Linked Inheritance?
  • X-linked inheritance refers to traits controlled by genes located on the X chromosome.
  • Since males have only one X chromosome (XY), they are more likely to express X-linked traits, even if the allele is recessive.
  • Females have two X chromosomes (XX) and usually need two copies of the recessive allele to express the trait.

Key Characteristics of X-Linked Inheritance

• Males are more affected because they have only one X chromosome (no backup gene on the Y chromosome).
• Females can be carriers if they have one normal allele and one defective allele (heterozygous).
• X-linked traits are passed from a carrier mother to her sons or daughters, or from an affected father to his daughters only.

Examples of X-Linked Recessive Disorders

• Hemophilia
  • A condition where the blood does not clot properly due to a lack of clotting factors.
  • Cause: Recessive allele on the X chromosome.
  • Males with one defective allele on the X chromosome will have hemophilia.
  • Females need two defective alleles to exhibit the condition.
• Red-Green Color Blindness
  • A condition where individuals cannot distinguish between red and green colors.
  • Cause: Recessive allele on the X chromosome.
  • Males with the defective allele are color blind.
  • Females can be carriers or, if homozygous recessive, color blind.

Hemophilia - Genotype and Phenotype

• X^N = Normal allele
• X^n = Diseased allele
• FEMALES:
  • X^NX^N = Normal
  • X^NX^n = Normal – carrier
  • X^nX^n = diseased
• MALES:
  • X^NY = Normal
  • X^nY = diseased

Test Cross - Punnett Square for X-Linked Inheritance

• Punnett Square for X-Linked Inheritance: Carrier Mother (X^NX^n) and Normal Father (X^NY):

• Percentage chance the child has the disease? 25%
• Percentage chance that a son has the disease? 50%

Test Cross - Punnett Square for X-Linked Inheritance

• Punnett Square for X-Linked Inheritance: Carrier Mother and Diseased Father:

• % chance that the child is diseased? 50%

Protein Synthesis

• During transcription, which takes place in the nucleus, a segment of DNA unzips, a complementary strand of mRNA is made.
• The mRNA then leaves the nucleus and binds to the ribosome in the cytoplasm.
• In translation, the ribosome reads the mRNA in codons (three-base sequences), and a specific tRNA molecules bring the correct amino acids to the ribosome that binds to the complimentary mRNA.
• The amino acids are linked together in the correct sequence to form a polypeptide chain.