Alleles, dominant and recessive, genotype and phenotype, working out crosses with Punnett squares and family trees, and how sex is determined.
You can study this deck without signing up — your progress is kept in this browser. An account saves it properly, syncs it across devices, and lets you edit the cards.
1.What is an allele?
A different form of the same gene.
2.What does homozygous mean?
Having two identical alleles for a gene, for example BB or bb.
3.What does heterozygous mean?
Having two different alleles for a gene, for example Bb.
4.What is a dominant allele?
An allele that is expressed even if only one copy is present. It is written with a capital letter.
5.What is a recessive allele?
An allele that is only expressed if two copies are present. It is written with a lower case letter.
6.What is a genotype?
The combination of alleles an organism has for a characteristic.
7.What is a phenotype?
The physical characteristic that is actually expressed.
8.Why can two individuals have the same phenotype but different genotypes?
A dominant characteristic shows whether the individual is homozygous dominant or heterozygous.
9.What is a Punnett square used for?
To work out the possible genotypes of offspring from a cross, and the probability of each.
10.In a cross between Bb and Bb, what ratio of phenotypes is expected?
Three showing the dominant characteristic to one showing the recessive, a 3 to 1 ratio.
11.In a cross between Bb and Bb, what is the probability of a homozygous recessive offspring?
One in four, which is 25 per cent.
12.In a cross between BB and bb, what are all the offspring?
All Bb, so all heterozygous and all showing the dominant characteristic.
13.In a cross between Bb and bb, what ratio is expected?
One to one. Half show the dominant characteristic and half the recessive.
14.Why are the results of a genetic cross only probabilities?
Fertilisation is random, so actual offspring may not match the predicted ratio, especially in small numbers.
15.What are the sex chromosomes in humans?
The 23rd pair. Females are XX and males are XY.
16.Which parent determines the sex of the child, and why?
The father. The mother can only pass on an X, while the father passes on either an X or a Y.
17.What is the probability of having a male child?
One in two, or 50 per cent.
18.How would you show sex determination in a Punnett square?
Cross XX with XY. The results are XX, XX, XY, XY, giving a one to one ratio of female to male.
19.What is a family tree diagram used for?
To trace how a characteristic or disorder has been inherited through the generations of a family.
20.How can you tell an allele is recessive from a family tree?
If two parents who do not show the characteristic have a child who does, the allele must be recessive and both parents carriers.
21.What is a carrier?
Someone who is heterozygous for a recessive disorder. They do not have the disorder themselves but can pass the allele on.
22.Most characteristics are controlled by what?
Multiple genes interacting, rather than a single gene.
23.Give three examples of characteristics controlled by a single gene.
Fur colour in mice, red-green colour blindness in humans, and the shape of a pea plant's seed pod.
24.Who was Gregor Mendel and what did he do?
A monk who carried out breeding experiments on pea plants in the nineteenth century and worked out the basic rules of inheritance.
25.What did Mendel conclude?
That characteristics are determined by separately inherited units, and that these units do not blend together.
26.Why was Mendel's work not recognised at the time?
Chromosomes had not yet been observed, so there was no known mechanism to explain his units of inheritance.
27.What happened in the early twentieth century that supported Mendel?
Chromosomes were observed behaving in ways that matched his units, and it was later shown that genes are located on chromosomes.
28.When was the structure of DNA determined?
In the 1950s, which finally explained how genes worked and led to the understanding of the genetic code.
29.If a plant with red flowers RR is crossed with a white flowered plant rr, what colour are the offspring?
All red, because all offspring are Rr and red is dominant.
30.Two heterozygous parents have four children. Must exactly one have the recessive condition?
No. One in four is a probability, not a guarantee. All four or none could have it.
31.How do you write the genotype of a person who is a carrier of a recessive disorder?
With one dominant and one recessive allele, for example Ff.
32.What is the difference between a gene and an allele?
A gene codes for a characteristic. An allele is one of the different versions of that gene.
You remember far more by trying to recall an answer than by reading it. Run these as flashcards and the ones you keep getting wrong will come back more often.
Start studying