Human Genetics

Human genetics is studied with pedigrees rather than experimental crosses, and autosomal and sex-linked inheritance differ. Autosomal dominant traits appear every generation, while recessive traits can appear in children of unaffected parents. X-linked recessive traits mostly affect sons, and traits like ABO blood type involve multiple alleles or many genes. Switch the inheritance-pattern slider and compare how the pedigree changes.

Why is human genetics complex?

👨‍👩‍👧‍👦 Features of Human Genetics

①Unlike peas, humans can't be experimentally crossed → pedigree analysis

②Autosomes (22 pairs) vs sex chromosomes (1 pair, XX/XY) inherit differently

③Polygenic (height, skin color): many genes → continuous variation

Pedigree Analysis

Inheritance pattern0

💡 Features by Pattern

①0=Autosomal dominant: appears every generation, equal in sexes

②1=Autosomal recessive: normal parents → 25% affected (higher with consanguinity)

③2=X-linked recessive: mostly sons affected, daughters may be carriers

Inheritance Pattern Comparison

Inheritance Patterns

ChartAutosomal vs Sex-Linked

Item	Autosomal Dominant	Autosomal Recessive	X-linked Recessive
Gene location	autosome	autosome	X chromosome
Expression	just one A	aa (rec. homozygous)	X^a Y (M) or X^a X^a (F)
Sex difference	none	none	males more affected
Examples	earlobes, dimples	albinism, sickle cell	color blindness, hemophilia

Human Inheritance Phenomena

Inheritance Types

Diverse Human Inheritance

Worked Examples

Example 1

A type-A father (genotype I^A i) and a type-B mother (genotype I^B i). List all possible blood types of their children.

The father’s gametes are I^A or i; the mother’s are I^B or i.

Combinations: I^A I^B (AB), I^A i (A), I^B i (B), ii (O) — each 1/4.

▸ A, B, AB, O (all four possible)

I^A and I^B are codominant, i is recessive. If both parents carry i, an O (ii) child is possible.

Example 2

A carrier mother (X^A X^a) for red-green color blindness (X-linked recessive) and a normal father (X^A Y). What is the chance of color blindness in sons and in daughters?

Daughters get X^A from the father, so they are X^A X^A or X^A X^a → all normal (0% color blind).

Sons get Y from the father and X from the mother → X^A Y (normal) or X^a Y (color blind), each 1/2.

▸ sons 1/2 color blind, daughters 0%

X-linked recessive: sons get only the mother’s X, so a carrier mother gives 1/2; daughters are spared by the father’s X^A (can be carriers).

Summary

Pedigree Test

normal parents → affected child = recessive / every gen = dominant

first step of pedigree analysis

Sex-linked Test

affected father → all daughters carriers (or affected) = X-linked

key clue for X-chromosome inheritance

CSAT-style

A carrier mother (X^A X^a) and a color-blind father (X^a Y) for red-green color blindness. What is the probability that a child is color blind?

①0

②14

③12

④34

⑤1

▸ ③ 12

Daughters: X^A X^a (carrier) or X^a X^a (color blind) → 1/2 of daughters affected. Sons: X^A Y (normal) or X^a Y (affected) → 1/2 of sons affected.

Both sons and daughters are 1/2 affected, so 1/2 of all children are color blind.

🎯 Exam Points

①Pedigree: dominant/recessive → autosomal/sex-linked in order

②Autosomal recessive: normal parents + affected child → Aa × Aa

③X-linked recessive: carrier mother → 50% sons affected

④ABO: I^A, I^B (codominant), i (recessive) → 6 genotypes

⑤Polygenic: continuous variation, normal distribution, large environmental effect

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Mendelian Genetics

Ecosystems & Energy Flow

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