Non-Mendelian Inheritance & Sex-linked Traits: Study Notes

Non-Mendelian Inheritance & Sex-linked Traits

Introduction

This guide covers patterns of inheritance that extend beyond simple Mendelian genetics, including incomplete dominance, co-dominance, multiple alleles, polygenic inheritance, and sex-linked traits. These concepts are essential for understanding the genetic diversity observed in populations and the inheritance of certain human diseases.

Mendelian Inheritance Review

Dihybrid Crosses

• Dihybrid Cross: A cross between two individuals heterozygous for two traits (e.g., RrYy x RrYy for pea seed shape and color).

• Traits: R = round, r = wrinkled, Y = yellow, y = green.

• Punnett Square: Used to predict offspring genotypes and phenotypes.

• Phenotypic Ratio: Classic dihybrid cross yields a 9:3:3:1 ratio (9 round/yellow : 3 round/green : 3 wrinkled/yellow : 1 wrinkled/green).

Example: Crossing RrYy x RrYy yields the following phenotypic ratio:

• 9 round/yellow

• 3 round/green

• 3 wrinkled/yellow

• 1 wrinkled/green

Gamete Formation (FOIL Method): Possible gametes from RrYy are RY, Ry, rY, ry.

Common Misconceptions about Dominance

• A dominant allele does not necessarily express a darker or bigger phenotype.

• A dominant allele does not mean it is more common in the population.

• A dominant allele does not mean it is advantageous or naturally selected for (e.g., Huntington's disease is dominant but not advantageous).

Non-Mendelian Inheritance Patterns

Incomplete Dominance

• Definition: Heterozygotes show an intermediate, blended phenotype.

• Example: In snapdragons, RR = red, rr = white, Rr = pink (blended color).

Co-dominance

• Definition: Both alleles affect the phenotype equally and separately; not blended.

• Example: Roan cattle have both red and white hairs; human MN blood group.

Multiple Alleles

• Definition: More than two alleles exist for a gene in the population.

• Example: Coat color in rabbits (C, cchd, ch, c).

Human ABO Blood Groups

• Example of: Co-dominance and multiple alleles.

• Alleles: IA, IB, i.

• IA and IB are co-dominant; i is recessive to both.

• Genotypes and phenotypes:

Genotype	Phenotype (Blood Group)	Antigen on RBC	Antibodies in Blood	Donation Status
IAIA or IAi	A	Type A	Anti-B	—
IBIB or IBi	B	Type B	Anti-A	—
IAIB	AB	Type A & B	None	Universal recipient
ii	O	None	Anti-A & Anti-B	Universal donor

Polygenic Inheritance

• Definition: Phenotype is determined by two or more genes, resulting in a continuous range of phenotypes.

• Examples: Human skin color, height, intelligence.

Sex-linked Traits

Chromosomes and Sex Determination

• Humans have 23 pairs of chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes (XX = female, XY = male).

• The Y chromosome contains the SRY gene, which triggers male development.

• The X chromosome carries many genes unrelated to sex determination.

X-linked Inheritance

• Definition: Traits determined by genes on the X chromosome.

• Examples: Hemophilia, red-green colorblindness, Duchenne muscular dystrophy.

• Inheritance Pattern: Males (XY) are more likely to express X-linked recessive traits because they have only one X chromosome.

Hemophilia Example

• Genotypes: XH = normal, Xh = hemophilia allele.

• Carrier female: XHXh

• Affected male: XhY

Punnett Square for Carrier Mother (XHXh) x Normal Father (XHY):

	XH	Y
XH	XHXH (normal daughter)	XHY (normal son)
Xh	XHXh (carrier daughter)	XhY (affected son)

Pedigree Analysis

• Pedigrees can be used to track inheritance of X-linked traits through families.

• Probability calculations involve multiplying independent probabilities (e.g., chance of being a carrier × chance of passing on the allele).

Review Questions

• How many phenotypes are possible for one gene in simple dominance?

• Can two brown-eyed parents have a blue-eyed child? Why or why not?

• What is the difference between polygenic traits and multiple alleles?

• Do homologous chromosomes always have the same alleles?

• What is the difference between a gene and a chromosome?

• How many chromosomes do humans have?

• What determines genetic sex in humans?

Summary Table: Key Inheritance Patterns

Pattern	Definition	Example
Incomplete Dominance	Heterozygote shows intermediate phenotype	Pink snapdragons (RR = red, rr = white, Rr = pink)
Co-dominance	Both alleles expressed equally	AB blood type
Multiple Alleles	More than two alleles for a gene	Rabbit coat color, ABO blood group
Polygenic Inheritance	Trait controlled by multiple genes	Skin color, height
Sex-linked Inheritance	Gene located on sex chromosome	Hemophilia, colorblindness

Additional info:

• Sex determination in mammals is more complex than just XX/XY; other genes and environmental factors can play a role.

• Pedigree analysis is a key tool in human genetics for tracking inheritance patterns and calculating probabilities.