Mendelian Genetics and Inheritance Patterns

Law of Segregation

• Each organism has two alleles for each gene (one from each parent)
• Alleles segregate during gamete formation — each gamete receives only ONE allele
• Basis: separation of homologous chromosomes in meiosis I

Law of Independent Assortment

• Genes on different chromosomes assort independently during meiosis
• Basis: random orientation of bivalents in metaphase I
• Exception: linked genes (on the same chromosome) tend to be inherited together

Complete Dominance

• Heterozygote shows dominant phenotype only
• Monohybrid cross (Aa × Aa): 3:1 phenotypic ratio
• Test cross (Aa × aa): 1:1 ratio confirms heterozygosity

Incomplete Dominance

• Heterozygote shows intermediate phenotype
• Example: Red × White snapdragons → Pink ($C^RC^W$)
• F₂ ratio: 1:2:1 (red : pink : white)

Codominance

• Both alleles fully expressed in heterozygote
• Example: ABO blood groups — $I^AI^B$ → Type AB (both A and B antigens present)
• Sickle cell: $Hb^AHb^S$ → both normal and sickle haemoglobin produced

Multiple Alleles

• More than two alleles exist in the population (individuals still have only 2)
• Example: ABO system has three alleles: $I^A$, $I^B$, $I^O$

• Genes on the X chromosome show different patterns in males and females
• Males ($XY$) express X-linked recessive alleles because they have only one X
• Example: Colour blindness ($X^bY$ = affected male; $X^BX^b$ = carrier female)
• Reciprocal crosses give different results for sex-linked traits

Epistasis

• One gene masks or modifies the expression of another
• Produces modified ratios (9:3:4, 12:3:1, 9:7, etc.)
• Example: Labrador coat colour — E gene controls pigment deposition; B gene determines colour

Polygenic Inheritance

• Multiple genes contribute to one phenotype
• Produces continuous variation (bell curve)
• Examples: height, skin colour, eye colour

Pleiotropy

• One gene affects multiple phenotypes
• Example: Sickle cell allele → sickle-shaped RBCs, anaemia, pain crises, malaria resistance

Environmental Effects

• Environment can influence gene expression
• Example: Hydrangea flower colour depends on soil pH; fur colour in Siamese cats depends on temperature

Autosomal Recessive

• Affected individuals often have unaffected parents (carriers)
• Appears in both sexes equally
• Skip generations pattern

Autosomal Dominant

• Affected individuals have at least one affected parent
• Does not skip generations (usually)

X-linked Recessive

• More males affected than females
• Affected father cannot pass to sons (sons get Y)
• Carrier mothers can have affected sons

$\chi^2 = \sum \frac{(O - E)^2}{E}$

Steps:

1. State null hypothesis ($H_0$): no significant difference between observed and expected
2. Calculate expected values from predicted ratio
3. Calculate $\chi^2$
4. Determine degrees of freedom = categories − 1
5. Compare to critical value (p = 0.05)
6. If $\chi^2$ < critical value → accept $H_0$ (results fit expected ratio)
7. If $\chi^2$ > critical value → reject $H_0$ (significant difference)

Example

Cross Aa × Aa, observe: 850 dominant, 150 recessive (total 1000) Expected (3:1): 750 dominant, 250 recessive $\chi^2 = \frac{(850-750)^2}{750} + \frac{(150-250)^2}{250} = \frac{10000}{750} + \frac{10000}{250} = 13.33 + 40 = 53.33$ df = 1; critical value = 3.84. Since 53.33 > 3.84 → reject $H_0$. Results do NOT fit a 3:1 ratio.

Question: In fruit flies, eye colour is X-linked. Red (R) is dominant over white (r). Cross a carrier female with a red-eyed male. What phenotypic ratio is expected? (3 points)

Solution:

Parents: $X^RX^r$ × $X^RY$

Expected ratio: 3 red : 1 white. All females are red-eyed. 50% of males are red-eyed, 50% are white-eyed.

• Mendel's laws: segregation (alleles separate in meiosis I) and independent assortment (genes on different chromosomes).
• Inheritance patterns: complete dominance (3:1), incomplete dominance (1:2:1), codominance, sex-linkage.
• Non-Mendelian: epistasis, polygenic inheritance, pleiotropy, environmental effects.
• Chi-squared test: $\chi^2 = \sum(O-E)^2/E$; compare to critical value to evaluate ratios.
• Pedigree analysis: identify autosomal dominant/recessive and X-linked patterns.