X-linked recessive disorders such as haemophilia appear more frequently in males because a male:

Has only one X chromosome, with no second allele to mask the recessive one

Cannot be a carrier of any gene

Always inherits the disease from the father

Down syndrome is caused by:

Trisomy of chromosome 21 (an extra 21st chromosome)

A recessive allele on the Y chromosome

In Mendel's dihybrid cross, the 9:3:3:1 ratio depends on the law of:

Independent assortment (genes on different chromosomes)

Principles of Inheritance and Variation — Mendel, the Ratios and Human Genetic Disorders

Genetics is where NEET separates students who can work a cross from those who only memorised ratios, so this chapter rewards understanding the logic behind each number. Start from Mendel: the 3:1 monohybrid and 9:3:3:1 dihybrid ratios are not facts to recall but consequences of two rules — segregation and independent assortment. Once those click, the "exceptions" (incomplete dominance, codominance, the modified 9:7 or 9:3:4 dihybrid ratios) become variations on the same machinery rather than a new list. The last block — sex determination and human genetic disorders — is largely recall, but even there the pedigree and the criss-cross of sex-linked inheritance repay being reasoned rather than memorised. Learn to draw the cross and the ratios take care of themselves.

Mendel's Laws and Ratios

Law of Dominance: in a heterozygote one allele (dominant) masks the other (recessive). Law of Segregation: the two alleles of a gene separate during gamete formation, so each gamete carries only one — this law has no exception.
Law of Independent Assortment: alleles of different genes assort independently — the basis of the 9:3:3:1 ratio — but this holds only for genes on different chromosomes. Linked genes break it.
A monohybrid cross (Tt × Tt) gives a 3:1 phenotypic ratio but a 1:2:1 genotypic ratio. A dihybrid cross (TtRr × TtRr) gives 9:3:3:1. A test cross (with the homozygous recessive) reveals an unknown genotype.

🎯 Why 3:1 in the phenotype but 1:2:1 in the genotype

The two ratios from the same cross confuse people, but they describe two different things. Cross Tt × Tt. Each parent makes two kinds of gamete, T and t, in equal numbers, so the offspring combinations are TT, Tt, tT and tt in equal quarters — that is the genotypic ratio 1 TT : 2 Tt : 1 tt, i.e. 1:2:1, because Tt and tT are the same genotype and add together. Now look at what those genotypes look like. TT and both Tt are all tall, because T is dominant and masks t; only tt is short. So three quarters are tall and one quarter short — the phenotypic ratio 3:1. The genotype ratio counts the allele combinations; the phenotype ratio counts the appearances, and the two differ only because the heterozygote looks like the dominant homozygote. Once you see that 3:1 is just 1:2:1 with the first three lumped together by dominance, you can generate either ratio from a Punnett square without memorising, and you can also see why incomplete dominance — where the heterozygote looks different — turns the phenotype ratio back into 1:2:1.

Beyond Simple Dominance

When dominance is not complete
Pattern	Heterozygote shows	Example
Incomplete dominance	An intermediate (blended) phenotype	Pink flowers in snapdragon (Antirrhinum)
Codominance	Both alleles fully and separately	AB blood group; both A and B antigens
Multiple alleles	More than two alleles for one gene in the population	ABO blood group (Iᴬ, Iᴮ, i)
Pleiotropy	One gene affecting many traits	Sickle-cell / phenylketonuria

⚠️ Incomplete dominance versus codominance — blended or both, never the same

These two are constantly swapped, yet the difference is sharp. In incomplete dominance the heterozygote is a blend — a genuinely intermediate phenotype that is neither parent, like the pink flower from a red and a white parent; the two alleles average out. In codominance both alleles are expressed fully and separately, not blended — the AB blood group shows both A antigen and B antigen at once, not some intermediate "AB-average" antigen. The test to apply: is the heterozygote something in between (incomplete dominance) or does it show both parental phenotypes side by side (codominance)? A pink flower is a blend, so incomplete dominance; AB blood carries two distinct antigens together, so codominance. Note also that in both cases the phenotypic ratio equals the genotypic ratio (1:2:1), because the heterozygote is now visibly different from either homozygote — a neat contrast with the 3:1 of simple dominance, and a favourite one-mark question.

Sex Determination and Human Disorders

Human sex is XX (female), XY (male); the sperm decides the sex because the male is heterogametic (produces X or Y). Sex-linked genes lie on the X chromosome, giving the criss-cross inheritance seen in haemophilia and colour blindness.
X-linked recessive disorders — haemophilia, colour blindness — appear far more in males, who need only one recessive allele (they have a single X). Autosomal recessive: sickle-cell anaemia, thalassaemia, phenylketonuria. Autosomal dominant: Huntington's.
Chromosomal disorders from non-disjunction: Down syndrome (trisomy of chromosome 21, 47 chromosomes), Klinefelter (XXY, male with some female traits), Turner (XO, sterile female).

🎯 Why sex-linked recessives fall so heavily on sons

Haemophilia and colour blindness run in a pattern that looks lopsided until you count the chromosomes. The genes sit on the X chromosome, and the recessive allele is hidden by a normal dominant one. A female has two X chromosomes, so a single faulty allele is masked by the healthy one on her other X — she is a carrier, unaffected. A male has only one X (his other sex chromosome is Y, which carries no matching allele), so a single recessive allele on his lone X has nothing to mask it and he is affected. That is why these conditions appear so much more often in males, and why the classic route is an unaffected carrier mother passing the allele to her sons. Reasoning from "how many X chromosomes, and is there a partner allele to hide behind" lets you predict any such pedigree, rather than memorising which relatives are affected — and it explains the criss-cross look, where the trait skips from grandfather through carrier daughter to grandson.

Key takeaways

Monohybrid Tt × Tt: 3:1 phenotype, 1:2:1 genotype. Dihybrid: 9:3:3:1. Test cross reveals an unknown genotype.
Segregation has no exception; independent assortment holds only for genes on different chromosomes.
Incomplete dominance = blended heterozygote (pink snapdragon); codominance = both alleles shown (AB blood).
In both incomplete dominance and codominance the phenotype ratio equals the genotype ratio (1:2:1).
Sex is XX (female) / XY (male); the sperm decides sex. ABO is multiple alleles; sickle-cell shows pleiotropy.
X-linked recessives (haemophilia, colour blindness) hit males more — they have only one X to mask the allele.
Sickle-cell, thalassaemia, PKU are autosomal recessive; Huntington's is autosomal dominant.
Down syndrome = trisomy 21 (47 chromosomes); Klinefelter = XXY; Turner = XO — all from non-disjunction.

Practice questions (8)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

A monohybrid cross Tt × Tt gives a phenotypic ratio of 3:1 but a genotypic ratio of:
1. 1:2:1
2. 3:1
3. 9:3:3:1
4. 1:1
Show answer
Answer: A — 1:2:1
The offspring are 1 TT : 2 Tt : 1 tt, i.e. 1:2:1 by genotype; the 3:1 phenotype arises only because TT and Tt look alike under dominance and are counted together. Choosing 3:1 for the genotype forgets that the two heterozygotes are genotypically distinct from the dominant homozygote.
Pink flowers in snapdragon from a red × white cross are an example of:
1. Incomplete dominance
2. Codominance
3. Complete dominance
4. Epistasis
Show answer
Answer: A — Incomplete dominance
The heterozygote is an intermediate blend (pink), neither red nor white, which is the hallmark of incomplete dominance. Codominance would show both parental colours together rather than a blended one, so the blended pink rules it out.
The AB blood group, in which both A and B antigens are fully expressed, illustrates:
1. Codominance
2. Incomplete dominance
3. Complete dominance
4. Pleiotropy
Show answer
Answer: A — Codominance
Both alleles are expressed fully and separately — A antigen and B antigen together — which is codominance, not a blend. If it were incomplete dominance the cell would show some single intermediate antigen instead of both, so the "both fully" wording pins it to codominance.
X-linked recessive disorders such as haemophilia appear more frequently in males because a male:
1. Has only one X chromosome, with no second allele to mask the recessive one
2. Has two X chromosomes
3. Cannot be a carrier of any gene
4. Always inherits the disease from the father
Show answer
Answer: A — Has only one X chromosome, with no second allele to mask the recessive one
A male's single X means a lone recessive allele has no partner allele on the Y to hide behind, so he is affected; a female's second X can mask it, making her a carrier. The classic route is inheritance from a carrier mother, not the father, so the last option also misstates the pattern.
Down syndrome is caused by:
1. Trisomy of chromosome 21 (an extra 21st chromosome)
2. An extra X chromosome (XXY)
3. A missing X chromosome (XO)
4. A recessive allele on the Y chromosome
Show answer
Answer: A — Trisomy of chromosome 21 (an extra 21st chromosome)
Down syndrome results from an extra copy of chromosome 21, giving 47 chromosomes in total. The XXY and XO options describe Klinefelter and Turner syndromes respectively, which are sex-chromosome disorders rather than autosomal trisomy 21.
In Mendel's dihybrid cross, the 9:3:3:1 ratio depends on the law of:
1. Independent assortment (genes on different chromosomes)
2. Segregation only
3. Linkage
4. Dominance only
Show answer
Answer: A — Independent assortment (genes on different chromosomes)
The 9:3:3:1 ratio arises because the two gene pairs assort independently, which needs them to be on different chromosomes. Linked genes are inherited together and would distort the ratio, so linkage is precisely the condition under which 9:3:3:1 breaks down.
A single gene affecting several apparently unrelated traits is an example of:
1. Pleiotropy
2. Multiple allelism
3. Codominance
4. Polygenic inheritance
Show answer
Answer: A — Pleiotropy
One gene influencing many traits is pleiotropy, as in phenylketonuria or sickle-cell disease. Polygenic inheritance is the reverse — many genes controlling one trait — so the two are easily swapped and the direction of the "one-to-many" is the key.
A test cross is performed by crossing an organism of unknown genotype with one that is:
1. Homozygous recessive
2. Homozygous dominant
3. Always heterozygous
4. Of a different species
Show answer
Answer: A — Homozygous recessive
Crossing with the homozygous recessive lets the recessive parent contribute only recessive alleles, so the offspring phenotypes directly reveal the unknown parent's gametes and hence its genotype. A homozygous dominant partner would mask the recessive contribution and defeat the purpose of the test.