An organism carrying two different alleles of a gene, written as Yy, is described as heterozygous for that gene.
Missing Word
heterozygous
Heterozygous means the two alleles of a gene differ; in such an organism a dominant allele hides a recessive one.
Category: Genetic Terms
Question 2
An organism's genetic makeup, including alleles that are not expressed, is its genotype.
Missing Word
genotype
The genotype is the set of alleles an organism carries, while its phenotype is what those alleles produce that can be observed.
Category: Genetic Terms
Question 3
The specific position of a gene on a chromosome is called its locus.
Missing Word
locus
A locus is a gene's address on a chromosome; alleles of a gene sit at the same locus on homologous chromosomes.
Category: Genetic Terms
Question 4
A cross between true-breeding parents that differ in only one characteristic is called a monohybrid cross.
Missing Word
monohybrid
A monohybrid cross follows a single gene; its F1 offspring are all heterozygous, and the F2 show a 3:1 phenotypic ratio.
Category: Monohybrid Crosses
Question 5
Mendel analyzed his pea counts using probability, a measure from 0 to 1 of how likely an event is to occur.
Missing Word
probability
Probability lets geneticists predict the expected frequency of offspring types, such as the 1 in 4 chance of a homozygous recessive offspring from two heterozygous parents.
Category: Monohybrid Crosses
Question 6
A cross that follows two different characteristics at once, such as seed color and seed texture, is called a dihybrid cross.
Missing Word
dihybrid
A dihybrid cross tracks two genes; when the F1 offspring are self-crossed and the genes assort independently, the F2 show a 9:3:3:1 phenotypic ratio.
Category: Dihybrid Crosses
Question 7
A Punnett square for a cross between two plants that each make four kinds of gametes contains 16 boxes.
Missing Word
16
Each of four gamete types from one parent pairs with each of four from the other, so 4 x 4 = 16 equally likely combinations, as in a cross between two RrYy plants.
Category: Dihybrid Crosses
Question 8
In snapdragons, crossing a red-flowered plant with a white-flowered plant produces offspring whose flowers are pink.
Missing Word
pink
Snapdragon flower color shows incomplete dominance: the heterozygote makes less red pigment, so its flowers are an intermediate pink.
Category: Dominance Variations
Question 9
When both alleles are fully expressed together in an individual carrying two different alleles, as in type AB blood, the pattern is called codominance.
Missing Word
codominance
In codominance, neither allele masks the other and both products appear; a person with one A and one B allele has both A and B markers on their red blood cells.
Category: Dominance Variations
Question 10
The tendency of genes located near each other on the same chromosome to be inherited together is called linkage.
Missing Word
linkage
Linked genes do not assort independently, so a dihybrid cross involving them does not give the 9:3:3:1 ratio unless crossing over separates them.
Category: Gene Interactions
Question 11
A trait such as human skin color that is controlled by several genes is described as polygenic.
Missing Word
polygenic
Polygenic traits depend on the combined effect of several genes, which is why traits such as human skin color and height show continuous variation.
Category: Gene Interactions
Question 12
Humans have 22 pairs of chromosomes that are not sex chromosomes, called autosomes.
Missing Word
autosomes
Autosomes are all the chromosomes other than the X and Y; humans have 22 pairs of them plus one pair of sex chromosomes.
Category: Sex-Linked Traits
Question 13
Because a male has only one X chromosome, he is described as hemizygous for genes found only on the X.
Missing Word
hemizygous
Hemizygous means having just one copy of a gene; a male expresses whatever allele his single X carries, whether dominant or recessive.
Category: Sex-Linked Traits
Question 14
A female with one copy of an X-linked recessive disease allele and one normal allele, who usually shows no symptoms, is called a carrier.
Missing Word
carrier
A carrier holds a hidden recessive allele and can pass it on; a carrier mother passes it to half of her sons, who are affected, and half of her daughters.
Category: Sex-Linked Traits
Question 15
An X-linked recessive disorder in which the blood does not clot normally is hemophilia.
Missing Word
hemophilia
Hemophilia is caused by a recessive allele on the X chromosome, which is why it occurs far more often in males.
Category: Sex-Linked Traits