Genetics problems

Yes. If a characteristic is controlled by one dominant and one recessive allele, both TT and Tt individuals show the dominant phenotype even though their genotypes differ. Only a tt individual shows the recessive phenotype, so a dominant phenotype is genotypically ambiguous without further testing.

From: Genotype vs Phenotype

Homozygous means an organism carries two identical alleles for a gene, either two dominant alleles (homozygous dominant, TT) or two recessive alleles (homozygous recessive, tt). Because both alleles are the same, all of its gametes carry that one allele for the gene.

From: Genotype vs Phenotype

It states that the two alleles of a gene in an organism separate from each other during gamete formation, so each gamete receives only one allele of the pair. When fertilisation occurs, the offspring gets one allele from each parent, restoring the pair.

From: Mendel's First Law (Law of Segregation)

During meiosis I, homologous chromosomes (which carry the two alleles of a gene) separate and move to opposite poles of the cell. Each gamete ends up with only one chromosome of the homologous pair, and therefore only one allele, which is the physical basis of segregation.

From: Mendel's First Law (Law of Segregation)

A heterozygous parent, such as Tt, produces two types of gametes in equal proportions, one carrying T and one carrying t, because the two alleles segregate during meiosis.

From: Mendel's First Law (Law of Segregation)

A Punnett square is a grid diagram used to predict all the possible genotype combinations of offspring from a cross, and hence their phenotype ratio. It works because each gamete carries only one allele of a gene, so combining a row gamete with a column gamete gives one possible offspring genotype.

From: Punnett Square Method

The number of boxes equals the number of gamete types from one parent multiplied by the number of gamete types from the other parent. A monohybrid cross between two heterozygotes needs a 2 x 2 grid (4 boxes), while a dihybrid cross between two double heterozygotes needs a 4 x 4 grid (16 boxes).

From: Punnett Square Method

First count how many boxes give each genotype, then group the genotypes that produce the same observable characteristic based on dominance. For example, in a Tt x Tt cross the genotype ratio is 1 TT : 2 Tt : 1 tt, but because TT and Tt both look tall, the phenotype ratio becomes 3 tall : 1 short.

From: Punnett Square Method

A monohybrid cross is a genetic cross that follows the inheritance of a single gene controlled by two alleles, one dominant and one recessive. It predicts the genotype and phenotype ratios of the offspring using a Punnett square.

From: Monohybrid Cross

A Tt x Tt cross gives a genotype ratio of 1 TT : 2 Tt : 1 tt among the offspring. Because TT and Tt both show the dominant phenotype, the phenotype ratio is 3 dominant : 1 recessive.

From: Monohybrid Cross

F1 is the first generation produced from crossing the original parental (P) generation, for example crossing TT x tt to get all Tt offspring. F2 is produced by crossing two F1 individuals together, such as Tt x Tt, and typically shows the recessive phenotype reappearing in a 3:1 ratio.

From: Monohybrid Cross

A 3:1 phenotype ratio appears when both parents are heterozygous for the gene, such as Tt x Tt. The genotype ratio 1 TT : 2 Tt : 1 tt becomes a 3:1 phenotype ratio because the dominant allele produces the same phenotype in both TT and Tt offspring.

From: Monohybrid Ratios: 3:1 vs 1:1

A 1:1 phenotype ratio appears when a heterozygous individual is crossed with a homozygous recessive individual, such as Tt x tt. Half the offspring inherit the dominant allele (Tt) and half inherit only recessive alleles (tt).

From: Monohybrid Ratios: 3:1 vs 1:1

A ratio such as 3:1 is a probability based on random combination of gametes at fertilisation, not a fixed outcome for a small number of offspring. With a small sample the actual numbers may differ from the expected ratio, though they tend to approach it more closely as the number of offspring increases.

From: Monohybrid Ratios: 3:1 vs 1:1

A test cross crosses an organism with a dominant phenotype but unknown genotype against a homozygous recessive individual. Because the recessive parent's genotype is certain (tt), the phenotype ratio of the offspring reveals whether the unknown parent was homozygous dominant or heterozygous.

From: Test Cross

A homozygous recessive organism (tt) can only produce one type of gamete, t. This means any dominant allele appearing in the offspring must have come from the unknown parent, so the offspring pattern directly reflects the unknown parent's genotype without any ambiguity from the tester's side.

From: Test Cross

Cross the unknown individual with a tt organism. If every offspring shows the dominant phenotype, the unknown parent is TT. If roughly half the offspring show the recessive phenotype (a 1:1 ratio), the unknown parent is Tt.

From: Test Cross

Incomplete dominance is a pattern of inheritance where neither allele is completely dominant over the other, so the heterozygote shows an intermediate phenotype that blends the effects of both alleles. A classic example is a cross between red and white four o'clock flowers, which produces pink flowers in the heterozygote.

From: Incomplete Dominance

Crossing two pink (C^R C^W) F1 plants gives an F2 genotype ratio of 1 C^R C^R : 2 C^R C^W : 1 C^W C^W. Because each genotype has its own distinct phenotype, the phenotype ratio is also 1 red : 2 pink : 1 white.

From: Incomplete Dominance

Since neither allele masks the other, both alleles are written as capital letters with a superscript to show which trait they represent, such as C^R and C^W, instead of one capital and one lower-case letter.

From: Incomplete Dominance

Codominance is a pattern of inheritance in which both alleles of a heterozygote are expressed fully and separately at the same time, rather than one masking the other or the two blending into an intermediate phenotype. Roan cattle coat colour and human AB blood group are classic examples.

From: Codominance

In incomplete dominance the heterozygote shows one blended, intermediate phenotype, such as pink flowers from red and white parents. In codominance, both parental traits appear fully and separately in the heterozygote, such as red and white hairs both present in a roan coat, or both A and B antigens present in AB blood.

From: Codominance

A person with genotype I^A I^B produces both the A antigen and the B antigen on their red blood cells simultaneously, giving blood group AB. Neither the I^A nor the I^B allele masks the other, so both are fully expressed together, which is the defining feature of codominance.

From: Codominance

Multiple alleles means that more than two alleles for a gene exist within a population, even though any individual organism can only carry two of them, one on each homologous chromosome. The ABO blood group gene, with alleles I^A, I^B and i, is a common example.

From: Multiple Alleles

Three alleles control the ABO blood group: I^A, I^B and i. I^A and I^B are codominant with each other, so a person with both shows blood group AB, while i is recessive to both, so it only shows as blood group O when present in two copies.

From: Multiple Alleles

Yes. Blood group A can result from the genotype I^A I^A (homozygous) or I^A i (heterozygous), because I^A is dominant over i in both cases. The only way to distinguish them is through further testing, such as the blood groups of their offspring.

From: Multiple Alleles

Three alleles control ABO blood group: I^A, I^B and i. I^A and I^B are codominant with each other, and both are dominant over the recessive allele i, giving four possible blood groups: A, B, AB and O.

From: ABO Blood Groups

Yes, if both parents are heterozygous carriers of i (genotypes I^A i and I^B i). Each parent can pass on an i allele, and a child who receives i from both parents will have genotype i i, giving blood group O.

From: ABO Blood Groups

Blood group AB comes only from genotype I^A I^B, because the I^A and I^B alleles are codominant and both are expressed together, producing both A and B antigens on the red blood cells at the same time.

From: ABO Blood Groups

It states that the alleles of one gene are distributed to gametes independently of the alleles of another gene, provided the genes are on different chromosomes. This means all combinations of alleles from different genes are equally likely to appear together in a gamete.

From: Mendel's Second Law (Law of Independent Assortment)

During meiosis I, each pair of homologous chromosomes separates independently of every other pair. Since different genes are usually located on different chromosome pairs, the separation of one gene's alleles does not affect the separation of another gene's alleles, which is the physical basis of independent assortment.

From: Mendel's Second Law (Law of Independent Assortment)

A TtYy individual produces four gamete types in equal proportion: TY, Ty, tY and ty. This is because the T/t alleles and the Y/y alleles assort independently of each other during meiosis.

From: Mendel's Second Law (Law of Independent Assortment)

A dihybrid cross is a genetic cross that follows two genes at the same time, each with two alleles, provided the genes are on different chromosomes. It predicts the genotype and phenotype ratios of offspring using a 4 x 4 Punnett square.

From: Dihybrid Cross

A TtYy x TtYy cross gives the classic dihybrid phenotype ratio of 9:3:3:1-9 showing both dominant traits, 3 showing the first dominant and second recessive trait, 3 showing the first recessive and second dominant trait, and 1 showing both recessive traits.

From: Dihybrid Cross

Each parent in a TtYy x TtYy cross produces four gamete types (TY, Ty, tY, ty) because the two genes assort independently. Combining four gamete types from one parent with four from the other gives 4 x 4, or 16, possible offspring combinations.

From: Dihybrid Cross

It comes from combining two independent monohybrid 3:1 ratios, one for each gene in a dihybrid cross. Multiplying (3/4 : 1/4) by (3/4 : 1/4) gives fractions of 9/16, 3/16, 3/16 and 1/16, which simplifies to the ratio 9:3:3:1.

From: How the 9:3:3:1 Dihybrid Ratio Is Derived

Because the two genes assort independently, the probability of an offspring inheriting a particular combination of traits from both genes is the product of the probability of each trait occurring on its own, not the sum. This is why the ratios are multiplied together.

From: How the 9:3:3:1 Dihybrid Ratio Is Derived

Only when both parents are heterozygous for both genes and the genes are on different chromosomes so they assort independently. A different pair of parental genotypes, such as a dihybrid test cross, produces a different ratio, such as 1:1:1:1.

From: How the 9:3:3:1 Dihybrid Ratio Is Derived

Sex is determined by one pair of sex chromosomes. Females have the genotype XX and males have the genotype XY. The presence of a Y chromosome, contributed by the father, triggers development as male.

From: Sex Determination

A mother (XX) can only produce eggs carrying an X chromosome, while a father (XY) produces X-carrying and Y-carrying sperm in equal numbers. Since fertilisation combines one egg with one sperm at random, there is an equal chance of producing an XX (female) or XY (male) offspring, giving a 1:1 ratio.

From: Sex Determination

The father determines the offspring's sex, because his sperm can carry either an X or a Y chromosome, while the mother's eggs can only carry an X chromosome. Which type of sperm fertilises the egg decides whether the offspring is XX (female) or XY (male).

From: Sex Determination

Sex-linked inheritance refers to genes carried on a sex chromosome, almost always the X chromosome. Because males (XY) and females (XX) carry different numbers of X chromosomes, the pattern of inheritance for these genes differs between the sexes.

From: Sex-Linked Inheritance

Males have only one X chromosome, so a single recessive allele on it is enough to produce the condition, since there is no second X chromosome to carry a masking dominant allele. Females have two X chromosomes, so they need two recessive alleles to show the condition, making them less often affected.

From: Sex-Linked Inheritance

A carrier is typically a female who is heterozygous for an X-linked recessive allele (e.g. X^H X^h). She does not show the condition because the dominant allele masks the recessive one, but she can pass the recessive allele to her offspring, who may be affected.

From: Sex-Linked Inheritance

Colour blindness is a recessive condition, and its gene is located on the X chromosome, making it an X-linked recessive trait. The allele for normal vision (X^N) is dominant over the allele for colour blindness (Xⁿ).

From: Colour Blindness

None of the daughters will be colour blind, since they always receive a normal X^N allele from their father, though half the daughters will be carriers (X^N Xⁿ). Half the sons will be colour blind (Xⁿ Y), because sons only inherit their single X chromosome from their mother.

From: Colour Blindness

Males have only one X chromosome, so a single Xⁿ allele is enough to cause colour blindness. Females have two X chromosomes, so they need two Xⁿ alleles to be colour blind; with only one Xⁿ allele they are unaffected carriers, which is why the condition appears far more often in males.

From: Colour Blindness

Haemophilia is caused by a recessive allele located on the X chromosome, making it an X-linked recessive disorder. The dominant allele (X^H) produces normal blood clotting, while the recessive allele (X^h) does not.

From: Haemophilia

Yes, if her father has haemophilia (X^h Y) and her mother is at least a carrier (X^H X^h) or affected (X^h X^h), a daughter can inherit an X^h allele from each parent and have genotype X^h X^h, meaning she has haemophilia.

From: Haemophilia

Crossing X^H X^h (carrier mother) with X^h Y (affected father) gives an offspring genotype ratio of 1 X^H X^h : 1 X^h X^h : 1 X^H Y : 1 X^h Y, so overall half of the children, both sons and daughters, are expected to have haemophilia.

From: Haemophilia

A pedigree diagram traces how a trait is passed through generations of a family, using symbols for males, females, and affected or unaffected individuals. It is used to deduce whether a trait is dominant or recessive, and whether its gene is autosomal or on the X chromosome.

From: Pedigree Analysis

The clearest sign is two unaffected parents producing an affected child. Since both parents show the dominant phenotype but their child shows the trait, both parents must be heterozygous carriers, meaning the trait is recessive.

From: Pedigree Analysis

X-linked recessive traits affect far more males than females, and importantly, an affected father never passes the trait to his sons (since sons get their only X chromosome from their mother). If this pattern holds throughout the pedigree, the trait is likely X-linked recessive rather than autosomal recessive.

From: Pedigree Analysis

Define the allele key, state the parental phenotypes and genotypes, show the gametes in circles, complete the Punnett square, list the offspring genotypes and phenotypes, and finally state the genotype and phenotype ratios.

From: Genetics Answering Technique

Circling the gametes is a standard convention that clearly separates the gamete stage (single alleles) from the parental genotype stage (allele pairs) and the offspring stage. Markers use it to check that gametes were derived correctly before checking the Punnett square.

From: Genetics Answering Technique

A genotype ratio counts every distinct genetic combination separately, such as 1 TT : 2 Tt : 1 tt. A phenotype ratio groups genotypes that produce the same observable trait, such as 3 tall (TT and Tt combined) : 1 short (tt). A full answer should give both when asked, and should not confuse one for the other.

From: Genetics Answering Technique

Continuous variation is typically shown as a bell-shaped, normal distribution curve, with most individuals clustered around an average value and progressively fewer individuals at the extremes.

From: Continuous vs Discontinuous Variation

Discontinuous variation is usually controlled by one or a small number of genes with clear-cut dominant and recessive effects, so environmental factors have little room to shift an individual from one category to another. Continuous variation depends on many genes acting together, and environmental factors such as diet or exercise can shift the value within the range.

From: Continuous vs Discontinuous Variation