Extensions and Modifications to Inheritance: Genetics Study Notes

Chapter 4: Extensions and Modifications to Inheritance

Outline

• Sex Determination

• Sex-Linked Characteristics/Sex Influence

• Single Locus Factors

• Gene Interaction

• Multifactorial/Complex Traits

Sex Determination

Chromosome Sex-Determination

Sex determination refers to the biological system that establishes the sexual phenotype of an organism. Various mechanisms exist across species, often involving specific sex chromosomes or genetic factors.

• Male Heterogamety (XX:XY system): In placental animals (including humans), males possess two different sex chromosomes (XY), while females have two of the same (XX).

• Female Heterogamety (ZW:ZZ system): In birds, females are heterogametic (ZW), and males are homogametic (ZZ).

• ZO Sex Determination: Some moths have only one sex chromosome (Z) in females, while males have two (ZZ).

• XO Sex Determination: In grasshoppers, males have only one sex chromosome (X), while females have two (XX).

• Ploidy Determines Sex: In some species, such as bees, sex is determined by the number of chromosome sets (haploid males, diploid females).

System	Male	Female	Example Species
XX:XY	XY	XX	Humans, mammals
ZW:ZZ	ZZ	ZW	Birds
ZO	ZZ	Z	Moths
XO	X	XX	Grasshoppers
Haplo-diploid	Haploid	Diploid	Bees

Additional info: The images show human, bird, and insect examples of these systems.

Genic and Environmental Sex Determination

Some species do not use sex chromosomes but rely on specific genes or environmental cues to determine sex.

• Genic Sex Determination: Sex is determined by particular genes, not chromosomes.

• Environmental Sex Determination: Factors such as temperature can influence sex determination (e.g., turtles, some reptiles).

Example: In turtles, incubation temperature during a critical period determines whether the embryo develops as male or female.

Sex-Linked Characteristics and Sex Influence

X-Linked Inheritance

X-linked traits are determined by genes located on the X chromosome. These traits often show distinct inheritance patterns due to the difference in sex chromosome composition between males and females.

• Essential X Chromosome: The X chromosome contains genes necessary for both sexes; at least one X is required for viability.

• Y Chromosome: The presence of a Y chromosome typically determines maleness due to the SRY gene.

• SRY Gene: The Sex-determining Region Y (SRY) gene triggers male development.

• Androgen-Insensitivity Syndrome: Caused by defective androgen receptors, leading to individuals with XY chromosomes developing female characteristics.

Example: Klinefelter syndrome (XXY) and Turner syndrome (XO) are human conditions resulting from atypical sex chromosome combinations.

Pedigree Patterns of X-Linked Traits

X-linked traits can be recessive or dominant, each showing characteristic patterns in family pedigrees.

• X-linked Recessive:

  • Trait is rare and often skips generations.

  • Affected fathers do not pass the trait to sons but can pass it to daughters (who may be carriers).

  • Males are more frequently affected than females.

• X-linked Dominant:

  • Trait is common and does not skip generations.

  • Affected fathers pass the trait to all daughters but not to sons.

  • Males and females are equally likely to be affected.

Inheritance Pattern	Key Features
X-linked Recessive	Skips generations, more males affected, affected fathers do not pass to sons
X-linked Dominant	Does not skip generations, affected fathers pass to all daughters

Example: Colorblindness is an X-linked recessive trait. If a colorblind man and a woman whose mother is colorblind have children, the probability of their child being colorblind depends on the mother's carrier status.

Reciprocal Crosses

Reciprocal crosses are used to determine if a trait is sex-linked by switching the sex of the parent carrying the trait.

• If the results differ depending on which parent carries the trait, the trait is likely sex-linked.

Example: White eyes in fruit flies were shown to be X-linked by reciprocal crosses.

Dosage Compensation and X-Inactivation

Dosage compensation ensures equal expression of X-linked genes in males and females. In mammals, one X chromosome in females is randomly inactivated, forming a Barr body.

• X-Inactivation: In females (XX), one X chromosome is condensed and inactivated in each cell.

• Mosaicism: Heterozygous females can show mosaic phenotypes, such as calico cats with patches of different fur colors.

Additional info: Other species use different dosage compensation mechanisms, such as upregulating the single X in males (Drosophila) or downregulating both Xs in hermaphroditic worms (C. elegans).

Single Locus Factors

Variations of Dominance

Not all alleles show complete dominance. Variations include incomplete dominance and codominance.

• Incomplete Dominance: The heterozygote displays a phenotype intermediate between the two homozygotes.

• Codominance: Both alleles are fully expressed in the heterozygote.

Example: In snapdragons, crossing red and white flowers produces pink offspring (incomplete dominance). In human blood type, both A and B alleles are expressed (codominance).

Penetrance and Expressivity

Penetrance and expressivity describe the relationship between genotype and phenotype.

• Penetrance: The percentage of individuals with a particular genotype who express the expected phenotype.

• Expressivity: The degree to which a trait is expressed among individuals with the same genotype.

Example: If 42 people have an allele for polydactyly and only 38 show extra fingers, the penetrance is $\frac{38}{42} \approx 90\%$.

Lethal Alleles

Lethal alleles cause death when present in certain genotypes, often affecting expected Mendelian ratios.

• Example: The Manx cat allele is lethal in homozygotes, resulting in a 2:1 ratio of Manx to normal kittens.

Multiple Alleles

Some genes have more than two alleles in the population, increasing genetic diversity.

• Example: The ABO blood group system in humans has three alleles: $I^A$, $I^B$, and $i$.

Genotype	Phenotype
$I^A I^A$, $I^A i$	Type A
$I^B I^B$, $I^B i$	Type B
$I^A I^B$	Type AB (codominant)
$ii$	Type O

Example: Paternity cases can use blood typing to exclude possible fathers.

Gene Interaction

Epistasis

Epistasis occurs when one gene masks the effect of another gene at a different locus.

• Recessive Epistasis: The recessive allele of one gene masks the expression of another gene.

• Dominant Epistasis: The dominant allele of one gene masks the expression of another gene.

Example: Labrador retriever coat color is determined by two genes: one for pigment color (B: black, b: brown) and one for pigment deposition (E: allows color, e: prevents color). The genotype $bb ee$ results in yellow fur regardless of the B gene.

Genotype	Phenotype
B_ E_	Black
bb E_	Brown
B_ ee or bb ee	Yellow

Complementation

Complementation tests determine whether mutations producing similar phenotypes are in the same or different genes.

• If two mutations are in different genes, crossing them can restore the wild-type phenotype (complementation).

• If mutations are in the same gene, the mutant phenotype persists.

Example: Flower color in sweet peas requires two genes; mutations in either gene can prevent pigment production.

Multifactorial/Complex Traits

Polygenic Traits

Polygenic traits are controlled by multiple genes, often resulting in continuous variation in phenotype.

• Quantitative Traits: Traits such as height, skin color, and weight are influenced by many genes and environmental factors.

• Additive Model: As the number of genes increases, the number of possible phenotypes increases, producing a bell-shaped distribution.

Example: Human skin color is determined by several genes and environmental factors.

Pleiotropy

Pleiotropy occurs when a single gene affects multiple, seemingly unrelated traits.

• Example: The gene for phenylketonuria (PKU) affects intellectual ability, skin color, and eye color.

Other Complex Inheritance Patterns

• Sex-Influenced Traits: Expression depends on the sex of the individual (e.g., pattern baldness).

• Sex-Limited Traits: Expressed in only one sex (e.g., milk production in females).

• Cytoplasmic Inheritance: Traits inherited through organelles such as mitochondria or chloroplasts, usually maternally.

• Genomic Imprinting: Expression depends on whether the allele is inherited from the mother or father.

• Conditional Traits: Expression depends on environmental conditions (e.g., temperature-sensitive alleles in Siamese cats).

• Phenocopy: Environmental factors produce a phenotype that mimics a genetic mutation.

Example: Mitochondrial diseases are inherited maternally; imprinted genes such as Igf2 show parent-of-origin effects.

Summary Table: Key Extensions to Mendelian Inheritance

Concept	Definition	Example
Incomplete Dominance	Heterozygote is intermediate	Pink snapdragons
Codominance	Both alleles expressed	AB blood type
Penetrance	% with genotype showing phenotype	Polydactyly
Expressivity	Degree of trait expression	Variable polydactyly
Lethal Alleles	Allele causes death	Manx cats
Multiple Alleles	More than two alleles	ABO blood group
Epistasis	Gene masks another gene	Labrador coat color
Polygenic Traits	Many genes affect trait	Height, skin color
Pleiotropy	One gene, multiple effects	PKU
Sex-Influenced	Expression depends on sex	Baldness
Sex-Limited	Trait in one sex only	Milk production
Cytoplasmic Inheritance	Non-nuclear genes	Mitochondrial diseases
Genomic Imprinting	Parent-of-origin effect	Igf2 gene
Conditional Traits	Environment-dependent	Siamese cat color
Phenocopy	Environment mimics mutation	Drug-induced defects