Difficulty: Intermediate | Prerequisites: Basic understanding of DNA, chromosomes, meiosis
This topic covers Mendelian genetics: how traits are inherited through dominant and recessive alleles, what happens when you cross organisms with known genotypes, and how chromosomal abnormalities arise. It is one of the most calculation-heavy parts of introductory biology, and the exam will expect you to work through Punnett squares and predict phenotypic ratios. You should already be comfortable with the basics of DNA, chromosomes and meiosis before working through this material.
Mendel's principles explain how traits pass from parents to offspring. A monohybrid cross between two heterozygotes (e.g. Ss x Ss) gives a 3:1 phenotypic ratio. A dihybrid cross between two double heterozygotes (e.g. TtYy x TtYy) gives a 9:3:3:1 ratio, illustrating independent assortment. Recessive disorders like Tay-Sachs persist in populations because carriers (heterozygotes) are unaffected. Chromosomal abnormalities such as Down syndrome result from nondisjunction during meiosis.
Dominant allele
The allele that is expressed in the phenotype when at least one copy is present. Represented by a capital letter (e.g. S for short hair).
Recessive allele
The allele that is expressed in the phenotype only when two copies are present (homozygous recessive). Represented by a lowercase letter (e.g. s for long hair).
Homozygous
Having two identical alleles for a given gene: homozygous dominant (SS) or homozygous recessive (ss).
Heterozygous
Having two different alleles for a given gene (e.g. Ss). The organism is a carrier of the recessive allele.
Genotype
The genetic makeup of an organism for a particular trait (e.g. Ss, TtYy). In simple terms, the letters.
Phenotype
The observable physical expression of the genotype (e.g. short hair, tall with yellow seeds). In simple terms, what the organism looks like.
Punnett square
A grid used to predict the genotypic and phenotypic ratios of offspring from a genetic cross.
Monohybrid cross
A cross examining the inheritance of a single trait (one gene, two alleles). Example: Ss x Ss.
Dihybrid cross
A cross examining the inheritance of two traits simultaneously. Example: TtYy x TtYy.
Phenotypic ratio
The ratio of different phenotypes among offspring. A monohybrid cross of two heterozygotes yields 3:1. A dihybrid cross yields 9:3:3:1.
Principle of Independent Assortment
Mendel's second law. Alleles for different traits segregate independently of one another during gamete formation, provided the genes are on different chromosomes (or far apart on the same chromosome).
F1 generation
The first filial generation, the offspring of the parental (P) cross.
F2 generation
The second filial generation, produced by crossing or self-fertilising the F1.
Carrier
A heterozygous individual who carries one copy of a recessive disease allele without showing symptoms. In simple terms, they have the gene but not the disease.
Tay-Sachs disease
A fatal autosomal recessive disorder. Affected individuals (homozygous recessive) lack a functional enzyme for lipid metabolism in the brain, leading to death in early childhood. The allele persists because carriers are unaffected.
Down syndrome (Trisomy 21)
A chromosomal abnormality caused by an extra copy of chromosome 21. Results from nondisjunction during meiosis.
Turner syndrome
A condition in which a female has only one X chromosome (45, X). Not trisomy 21.
Klinefelter syndrome
A condition in which a male has an extra X chromosome (47, XXY). Not trisomy 21.
Edward syndrome (Trisomy 18)
A chromosomal abnormality caused by an extra copy of chromosome 18. Distinct from Down syndrome (chromosome 21).
Example from the exam: in cats, short hair (S) is dominant to long hair (s). Cross two heterozygotes: Ss x Ss.
Punnett square yields: 1 SS, 2 Ss, 1 ss.
Genotypic ratio: 1:2:1. Phenotypic ratio: 3 short hair : 1 long hair (because both SS and Ss display the dominant phenotype).
Mr and Mrs Smith both have widow's peaks (W is dominant). Their first child has a peak; their second does not.
Both parents must be heterozygous (Ww), because they express the dominant trait but can produce a child without it.
The second child is homozygous recessive (ww), having inherited one recessive allele from each parent.
A Ww x Ww cross gives a 25% chance of ww offspring.
Example from the exam: tall (T, dominant) vs dwarf (t), yellow seeds (Y, dominant) vs green seeds (y). Cross TTYY x ttyy.
F1 generation: all TtYy (tall with yellow seeds). Every offspring is heterozygous for both traits.
F2 generation (TtYy x TtYy self-fertilisation): phenotypic ratio is 9:3:3:1
9 tall, yellow
3 tall, green
3 dwarf, yellow
1 dwarf, green
Mendel's second law states that alleles for different genes segregate independently during gamete formation. The 9:3:3:1 ratio in a dihybrid cross is the direct result of this principle. It holds when genes are on different chromosomes or far apart on the same chromosome.
Tay-Sachs disease is an autosomal recessive disorder, fatal in early childhood. The allele persists in the population because heterozygous carriers (one normal allele, one disease allele) are healthy and show no symptoms. They can pass the recessive allele to offspring without knowing they carry it.
Down syndrome (Trisomy 21): three copies of chromosome 21 instead of two. Caused by nondisjunction during meiosis.
Edward syndrome (Trisomy 18): three copies of chromosome 18.
Turner syndrome (45, X): a female with only one X chromosome.
Klinefelter syndrome (47, XXY): a male with an extra X chromosome.
The exam specifically tests Trisomy 21 = Down syndrome. Do not confuse it with Edward syndrome (chromosome 18).
Genetic carrier screening (e.g. for Tay-Sachs, cystic fibrosis, sickle cell) uses exactly these inheritance principles. Couples who are both carriers of a recessive disease have a 25% chance per pregnancy of having an affected child.
Students often think that a 3:1 ratio means exactly 3 out of 4 offspring will show the dominant trait. It is a probability, not a guarantee. Each individual offspring has a 75% chance of expressing the dominant phenotype.
Students sometimes confuse genotype and phenotype. Two organisms can have different genotypes (SS and Ss) but the same phenotype (short hair).
Students often think Tay-Sachs persists because of new mutations. The main reason is that heterozygous carriers are healthy and unknowingly pass the allele on.
Students sometimes confuse Down syndrome (trisomy 21) with Edward syndrome (trisomy 18). Remember: Down = 21.
The 3:1 phenotypic ratio from a monohybrid heterozygous cross is a guaranteed exam topic.
Be ready to work through a dihybrid cross and explain the 9:3:3:1 ratio with the principle of independent assortment.
Expect a question explaining why recessive lethal alleles persist in the gene pool (carriers are unaffected).
Know which trisomy corresponds to which syndrome (21 = Down, 18 = Edward).
The widow's peak problem (both parents Ww, child ww) is a classic short-answer question.
Fill in the blank: A cross between Ss and Ss produces a phenotypic ratio of ________. (3:1.)
True or False: Down syndrome is caused by trisomy 18. (False, it is trisomy 21.)
Fill in the blank: The F1 offspring of TTYY x ttyy are all ________. (TtYy.)
True or False: Carriers of Tay-Sachs disease show symptoms of the disease. (False, carriers are heterozygous and unaffected.)
Fill in the blank: The phenotypic ratio of a dihybrid cross between two heterozygotes is ________. (9:3:3:1.)
Q: In cats, short hair (S) is dominant to long hair (s). If two heterozygous cats (Ss) are crossed, what is the expected phenotypic ratio?
A: 3 short hair : 1 long hair. The Punnett square gives 1 SS, 2 Ss and 1 ss. Both SS and Ss express short hair.
Q: Why does the allele for Tay-Sachs disease remain in the gene pool despite being fatal in early childhood?
A: Tay-Sachs is autosomal recessive. Heterozygous carriers (one normal allele, one disease allele) do not express the disease and are healthy. They can pass the recessive allele to future generations without any selective disadvantage.
Q: Which chromosomal abnormality is characterised by an extra chromosome on the 21st pair?
A: Down syndrome (Trisomy 21).
Q: Mr and Mrs Smith both have widow's peaks (dominant trait, W). Their second child lacks the peak. Explain the genotypes.
A: Both parents must be heterozygous (Ww). They each carry one dominant (W) and one recessive (w) allele. The second child inherited the recessive allele from each parent, giving a genotype of ww (homozygous recessive), which does not express the widow's peak. There is a 25% probability of this outcome in any pregnancy.
Q: A scientist crosses TTYY (tall, yellow) with ttyy (dwarf, green). What are the F1 genotype/phenotype and the F2 phenotypic ratio?
A: F1 genotype is TtYy. Phenotype is tall with yellow seeds (both dominant traits expressed). When F1 self-fertilises, the F2 phenotypic ratio is 9:3:3:1 (9 tall yellow, 3 tall green, 3 dwarf yellow, 1 dwarf green). This follows the principle of independent assortment, where alleles for height and seed colour segregate independently during gamete formation.
Genetics connects to biotechnology (the next set of notes), where techniques like gene guns and tissue culture are used to manipulate inherited traits. It also connects back to cell biology: chromosomal abnormalities arise from errors in meiosis (nondisjunction), and understanding DNA replication and the central dogma is essential for grasping how genes are expressed.
genetics, Mendelian genetics, inheritance, dominant, recessive, allele, homozygous, heterozygous, genotype, phenotype, Punnett square, monohybrid cross, dihybrid cross, phenotypic ratio, 3:1 ratio, 9:3:3:1 ratio, independent assortment, segregation, F1 generation, F2 generation, carrier, autosomal recessive, Tay-Sachs disease, Down syndrome, trisomy 21, trisomy 18, Edward syndrome, Turner syndrome, Klinefelter syndrome, nondisjunction, widow's peak, BIOL 110, intro to biology, Penn State