Biology

Inheritance & Variation of Traits

Punnett Square

diagram showing the gene combinations that might result from a genetic cross

monohybrid cross

a cross that involves one pair of contrasting traits

dihybrid cross

a cross involving two traits

allele

different forms of a gene

gametes

reproductive cell

somatic cells

any cells in the body other than reproductive cells

phenotype

an organism's physical appearance, or visible traits.

genotype

an organism's genetic makeup, or allele combinations.

dominant trait

a genetic factor that blocks another genetic factor

recessive trait

a trait that is apparent only when two recessive alleles for the same characteristic are inherited

heterozygous (hybrid)

having two different alleles for a trait

homozygous (purebred)

having two identical alleles for a trait

codominant

two genetic traits are fully expressed and neither dominates

incomplete dominance

one allele is not completely dominant over another allele

sex-linked traits

traits that are inherited with sex chromosomes

mendelian trait

a trait completely determined by a single gene

non mendelian trait

traits that are not determined by dominant and recessive alleles and can involve more than one gene

independent assortment

the random distribution of genes on different chromosomes to the gametes

crossing over

the exchange of genes between homologous chromosomes, resulting in a mixture of parental characteristics in offspring.

meiosis

a type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell

haploid (n)

having a single set of unpaired chromosomes

diploid (2n)

contain 2 copies of each chromosome

autosome

any chromosome that is not a sex chromosome

homologous chromosomes

pair of chromosomes that are the same size, same appearance and contain the same genes

sister chromatids

replicated forms of a chromosome joined together by the centromere and eventually separated during mitosis or meiosis II.

phenotypic ratio

the ratio of phenotypes that could appear in offspring

genotypic ratio

the ratio of the genotypes that appear in offspring

chromosome

a threadlike, gene-carrying structure found in the nucleus

law of segregation

pairs of alleles for a trait separate when gametes are formed during meiosis

chromatid

one half of the chromosome

autosome

chromosome that is not a sex chromosome

Meiosis I

The first division of a two-stage process of cell division that results in cells with half the number of chromosomes as the original cell; crossing over occurs in Meiosis I

Meiosis II

the second phase of meiosis consisting of chromatids separating, along with the two diploid cells splitting in two

F1 generation

the first generation of offspring obtained from an experimental cross of two organismssexual reproduction (meiosis)

sexual reproduction (meiosis)

one division twice to make four genetically different cells

all 4 cells are haploid cells (n) meaning they have 1/2 of the chromosomes of the parent cells

makes gametes