Monohybrid inheritance Flashcards – Cambridge IGCSE (9–1) Biology 0970 (Extended) (2026–2028)

What is inheritance?

The transmission of genetic information from one generation to the next.

What does an organism inherit from its parents?

Alleles of genes.

What is an organism’s genotype?

Its genetic make-up, including the alleles it has.

What is an organism’s phenotype?

Its observable features.

What is the difference between homozygous and heterozygous genotypes for a gene?

A homozygous genotype has two identical alleles; a heterozygous genotype has two different alleles.

Why are two individuals with the same homozygous genotype pure-breeding for that gene?

Both pass on the same allele of the gene.

Why is a heterozygous individual not pure-breeding for a gene?

It can pass on either of its different alleles.

When is a dominant allele expressed in the phenotype?

Whenever it is present in the genotype.

When is a recessive allele expressed in the phenotype?

Only when no dominant allele of that gene is present.

What does a monohybrid cross predict, and how?

It predicts inheritance of one characteristic by combining the alleles each parent can pass on to find possible offspring genotypes.

How does a Punnett square show possible offspring genotypes?

Possible parental alleles are placed along the edges; each cell shows a possible offspring genotype.

How are offspring genotypes used to find a phenotypic ratio?

Use the genotypes to predict the offspring phenotypes, then calculate their ratio.

With one dominant and one recessive allele, what phenotypic ratio is predicted from crossing two heterozygous parents?

3:1 dominant to recessive.

With one dominant and one recessive allele, what phenotypic ratio is predicted from crossing a heterozygous parent with a homozygous recessive parent?

1:1 dominant to recessive.

What does a pedigree diagram show about a family characteristic?

Which family members show or lack the characteristic across generations.

How are individuals and family relationships represented in a pedigree?

By connected symbols.

How can a pedigree be used to interpret a characteristic’s inheritance pattern?

Trace the characteristic from parents to offspring through successive generations.

What does a test cross determine about an individual with a dominant phenotype?

Whether the individual is homozygous dominant or heterozygous.

What type of individual is crossed with the individual being tested in a test cross?

A homozygous recessive individual, which can pass on only the recessive allele.

What does a recessive-phenotype offspring reveal about the individual being tested in a test cross?

The tested individual is heterozygous: it must have contributed a recessive allele.

What offspring phenotypes are predicted if the individual tested in a test cross is homozygous dominant?

All offspring are predicted to show the dominant phenotype.

How does a heterozygote’s phenotype differ in codominance and a dominant–recessive relationship?

In codominance, both alleles contribute to the phenotype; neither completely masks the other. In a dominant–recessive relationship, the dominant allele determines the phenotype.

What are the three alleles that determine ABO blood group?

$I^{A}$, $I^{B}$ and $I^{O}$.

How many ABO alleles does each individual inherit?

Two.

What is the dominance relationship between $I^{A}$ and $I^{B}$?

They are codominant: together they produce blood group AB.

How do $I^{A}$ and $I^{B}$ each relate to $I^{O}$ in dominance?

Each is dominant over $I^{O}$.

Which two genotypes produce blood group A?

$I^{A}I^{A}$ and $I^{A}I^{O}$.

Which two genotypes produce blood group B?

$I^{B}I^{B}$ and $I^{B}I^{O}$.

Which genotype produces blood group O?

$I^{O}I^{O}$.

Does an ABO blood-group phenotype always identify a unique genotype?

No. Some blood groups can result from more than one genotype.

What determines a sex-linked characteristic?

A gene on a sex chromosome.

How many copies of an X-linked gene do females and males usually have?

Females usually have two; males have one, as their other sex chromosome is a Y.

How does expression of a recessive X-linked allele differ between a male with one copy and a female with one recessive and one dominant allele?

The male can express the recessive characteristic; the female usually does not.

What is an example of a recessive X-linked characteristic?

Red-green colour blindness.

How does a genetic diagram predict offspring genotypes for an X-linked characteristic?

Place each allele on the X chromosome carrying it, then combine the sex chromosomes passed on by the parents.

Which sex chromosomes do parents pass to their daughters and sons?

The mother passes an X to every child. The father passes an X to a daughter and a Y to a son.

Why are daughters’ and sons’ phenotypes determined separately in an X-linked cross?

A male’s single X-linked allele can be expressed without a second allele of that gene.

How is the phenotypic ratio predicted from an X-linked cross?

Count the predicted offspring phenotypes, including any differences between the sexes.