Inheritance Flashcards

All 50 cards in this deck

What organism did Gregor Mendel use in his breeding experiments, and what did he investigate?

Pea plants; he investigated how characteristics such as seed shape, flower colour and plant height are passed to offspring.

What did Mendel observe when he crossed pure-breeding round-seeded pea plants with wrinkled-seeded pea plants, then bred those offspring together?

The first generation were all round-seeded; the next generation were round : wrinkled in a 3 : 1 ratio.

What did Mendel conclude about how characteristics are inherited?

Characteristics are controlled by pairs of separate 'units of inheritance', one inherited from each parent, and one unit can be dominant over the other.

What are Mendel's 'units of inheritance' now called?

Genes — with different versions of a gene called alleles.

Why was Mendel's work not accepted by other scientists at the time?

Chromosomes, genes and DNA had not yet been discovered, so there was no known mechanism to explain his units of inheritance; he was also little known and his work was not widely read.

True or false? Mendel's ideas about inheritance were recognised as important during his lifetime.

False. They were only accepted decades after his death, once chromosomes and genes had been discovered.

What is an allele?

An alternative version (form) of a gene.

What is a phenotype?

The observable characteristics (traits) expressed by an organism.

What is a genotype?

The combination of alleles an organism has for a characteristic.

What is the difference between a homozygous and a heterozygous genotype?

Homozygous = two identical alleles of a gene (e.g. $AA$ or $aa$); heterozygous = two different alleles (e.g. $Aa$).

What is the difference between a dominant and a recessive allele?

A dominant allele is expressed in the phenotype when only one copy is present; a recessive allele is only expressed when two copies are present.

What is a zygote?

The single cell formed when two gametes (sex cells) fuse at fertilisation.

What are the steps to complete a Punnett square for a monohybrid cross?

e.g. $Aa \times Aa$

1. Write one parent's alleles along the top and the other's down the side: $A$, $a$ and $A$, $a$.
2. Fill each box with the row allele + column allele: $AA$, $Aa$, $Aa$, $aa$.
3. Read off the phenotypes: 3 dominant : 1 recessive.

What are the steps to give the outcome of a monohybrid cross as a probability and a percentage?

e.g. a Punnett square giving $AA$, $Aa$, $Aa$, $aa$

1. Count the boxes with the phenotype you want: 1 box out of 4 is $aa$.
2. Write it as a probability: $1/4$ (1 in 4).
3. Convert to a percentage: 25%.

What phenotype ratio is expected when two heterozygous parents are crossed?

e.g. $Aa \times Aa$

3 dominant : 1 recessive — so 25% (1 in 4) show the recessive phenotype.

What is a carrier of a genetic disorder?

A heterozygous person who has one recessive disorder allele, so does not have the disorder but can pass the allele to offspring.

True or false? A Punnett square tells you exactly how many offspring of each type will be produced.

False. It gives the probability (expected ratio) of each genotype and phenotype, not the actual numbers.

Which sex chromosomes do human females and males have?

Females are XX; males are XY.

What are the steps to use a genetic diagram to show how sex is determined in humans?

e.g. XX mother $\times$ XY father

1. Write the parents' gametes: mother X and X; father X and Y.
2. Combine them in the Punnett square: XX, XX, XY, XY.
3. Read the outcome: 2 female : 2 male, i.e. a 50% chance of each.

Which parent determines the sex of a child, and why?

The father — his sperm carry either an X or a Y chromosome, while all the mother's eggs carry an X.

What is the probability that a human baby is male?

50% (1 in 2) — a 1 : 1 ratio of males to females.

True or false? Every human egg cell contains an X chromosome.

True. The mother is XX, so all her eggs carry an X chromosome.

True or false? A couple who already have three sons are more likely to have a daughter next.

False. Each fertilisation is independent, so the chance is still 50% male and 50% female.

What does codominant mean?

Both alleles are expressed in the phenotype of a heterozygous individual; neither is recessive.

What is meant by multiple alleles?

A gene that has more than two different alleles in the population, e.g. the three ABO blood group alleles.

Which alleles control the ABO blood groups, and how do they relate to each other?

Three alleles: $I^A$ and $I^B$, which are codominant with each other, and $I^O$, which is recessive to both.

Which genotypes give blood group A?

$I^AI^A$ or $I^AI^O$.

Which genotype gives blood group O?

$I^OI^O$ — two copies of the recessive allele.

True or false? In a person with blood group AB, one allele is dominant over the other.

False. $I^A$ and $I^B$ are codominant, so both are expressed and the person has both antigens.

What is a sex-linked characteristic?

A characteristic controlled by a gene carried on a sex chromosome, usually the X chromosome.

Why are males more likely than females to be affected by an X-linked recessive disorder?

Males have only one X chromosome, so a single recessive allele is expressed; females would need the recessive allele on both X chromosomes.

Name two examples of sex-linked recessive genetic disorders in humans.

Haemophilia and Duchenne muscular dystrophy (also red–green colour blindness).

How are the genotypes of a female carrier and an affected male for haemophilia written?

Carrier female $X^HX^h$; affected male $X^hY$.

What are the steps to find the chance of an affected child from a carrier mother and an unaffected father for an X-linked recessive disorder?

e.g. $X^HX^h \times X^HY$

1. Write the gametes: mother $X^H$ and $X^h$; father $X^H$ and Y.
2. Fill the Punnett square: $X^HX^H$, $X^HX^h$, $X^HY$, $X^hY$.
3. Count affected offspring: 1 in 4 (25%) — an affected son.

True or false? A male can be an unaffected carrier of an X-linked recessive disorder.

False. He has only one X chromosome, so if he has the recessive allele he is affected.

What is polygenic inheritance?

Inheritance of a characteristic that is controlled by more than one gene.

Are most human phenotypic features controlled by one gene or by many genes?

By many genes — most phenotypic features are polygenic rather than single-gene inheritance.

Give two examples of human characteristics that are polygenic.

Height and body mass (also skin colour and eye colour).

True or false? Human eye colour is controlled by a single gene with brown dominant to blue.

False. Eye colour is polygenic — it is controlled by several genes.

What is variation?

Differences in the characteristics of individuals of the same species.

What are the two types of cause of variation in phenotype?

Genetic variation (from mutation and sexual reproduction) and environmental variation (acquired characteristics).

What is an acquired characteristic?

A characteristic caused by the organism's environment rather than by its alleles, e.g. a scar or the language a person speaks.

Give an example of a human characteristic influenced by both genes and the environment.

Body mass — affected by inherited alleles and by diet, exercise and disease (height is another example).

What was the main outcome of the Human Genome Project?

The complete base sequence of the human genome was worked out, and the positions of human genes were mapped.

Give two potential medical applications of the Human Genome Project.

Predicting a person's risk of a disease and testing for genetic disorders; also developing new drugs and personalised (tailored) medicines.

What is a mutation?

A random change in the base sequence of DNA, which can produce a new allele of a gene.

How does genetic variation within a population of a species arise?

Through mutations, which produce new alleles; these are then spread and combined by sexual reproduction.

What are the three possible effects a mutation can have on the phenotype?

Most have no effect; some have a small effect; rarely, a single mutation significantly affects the phenotype.

True or false? Most mutations change an organism's phenotype.

False. Most mutations have no effect on the phenotype at all.

True or false? There is usually extensive genetic variation within a population of a species.

True. Mutations over time produce many different alleles within the population.