Heredity - Part II

Nondisjunction

error in meiosis or mitosis in which members of a pair of homologous chromosomes or a pair of sister chromatids fail to separate properly from each other

n + 1

gametes that form with an extra chromosome; resulting from nondisjunction during meiosis

n - 1

gametes that form with a missing chromosome; resulting from nondisjunction during meiosis

Polysomy

type of chromosomal mutation when there is too many copies of a particular chromosome

Trisomy 21

condition in which an individual has three copies of a chromosome, resulting in Down Syndrome

Monosomy

condition that occurs when an organism is missing a chromosome

Karyotype

picture of all the chromosomes in a cell arranged in pairs

Polyploidy

condition in which an organism has extra sets of chromosomes

Seedless Plants

varieties of plants that develop when an organism is triploid or has an odd number of chromosome sets

Heredity

passing of traits from parents to offspring, often demonstrating patterns of inheritance

P generation

first generation of parent organisms in a study

F generation

subsequent generations in a family of organisms, such as F1 and F2

Genome

complete genetic information of an organism

Genotype

combination of alleles an organism has for a specific characteristic within its genome

Gene

section of the genome that codes for a particular trait

Allele

version of a gene found at a specific location in the DNA

Locus

specific place on the chromosome where an allele is located within the DNA

Law of Dominance

when two different alleles are present in a genotype, only the dominant allele is expressed

Dominant Allele

trait that is expressed when there is at least one copy

Recessive Allele

trait that is only expressed when it is the only allele present in the genotype (both alleles must be the same)

True-Breeding Cross

when purebred organisms that contain genotypes of a single allele type are mated with one another

Purebred

an organism that only has two of the same allele in its genotype

Law of Segregation of Alleles

alleles of the same gene do not travel together when chromosomes are placed into gametes

Monohybrid Cross

occurs when two hybrid parents, each possessing two different alleles for a specific gene, are mated

Punnett Square

tool used to predict offspring genotypes based on parent genotypes

Homozygous

having two of the same allele

Heterozygous

having two different alleles

Phenotype

expression of a genetic trait, such as physical appearance

Test Cross

used to determine the unknown genotype of an organism displaying a dominant phenotype

Homozygous Dominant

genotype of the parent in a test cross when all offspring show the dominant phenotype

Heterozygous

genotype of the parent in a test cross when some of the offspring show the dominant phenotype and some of the offspring show the recessive phenotype

Dihybrid Cross

occurs when two parents that are both heterozygous for two specific traits are mated

2^n, where n is the number of traits that have two options

formula used to determine the number of possible allele combinations in a gamete

Phenotypic Ratio

relative frequency or amount of each observable phenotype in the offspring

Non-Mendelian Inheritance

occurs when observed phenotypic ratios deviate from expected ratios

Simple (Complete) Dominance

one allele is completely dominant and the other is recessive

3:1

observed ratio of dominant to recessive phenotypes in a monohybrid cross

9:3:3:1

observed ratio in a dihybrid cross, showing more offspring with both dominant phenotypes and the least offspring with both recessive phenotypes

1:1:1:1

observed ratio in a dihybrid test cross, showing equal amounts of offspring of both dominant phenotypes as offspring with both recessive phenotypes

Multiplication Rule of Probability

used to calculate the likelihood of combined events, such as having two different phenotypes

Epistasis

one gene controls the expression of another gene

Codominance

both alleles are fully expressed and contribute to the phenotype of a heterozygous organism

Incomplete Dominance

heterozygous organisms show a blending of two alleles in their phenotype

1:2:1

phenotypic ratio seen when a monohybrid cross is performed for a trait with incomplete dominance or codominance

Addition Rule

used when considering the likelihood of getting one phenotype or another

Mutually Exclusive Traits

characteristics that cannot both occur at that same time, such as dominant and recessive phenotypes for a trait

Multiple Allele Trait

occurs when there are more than two possible alleles to choose from, such as blood types

Rh factor

characteristic of blood type that describes the presence of a glycoprotein found on red blood cells

Gene Linkage

occurs when two different genes are located on the same chromosome and are inherited together

Crossing Over

meiotic process that can break the linkage between two genes

Independent Assortment

random distribution of alleles among gametes that is unable to occur when genes are linked

Autosomal Trait

found on chromosomes that are common to males and females; characteristic that appears equally in males and females

Sex-Linked Trait

found on the last pair of chromosomes; most commonly X-linked (not

found on the Y chromosome)

X-Linked Trait

shows different inheritance patterns

for males and females

Pedigree

diagram that shows inheritance of a particular trait through a family; allows us to determine whether a trait is dominant/recessive and autosomal/sex-linked

Recessive Inheritance Pattern

often shows that a trait can be hidden

Dominant Inheritance Pattern

often appears in each generation of a family and cannot be hidden

Sex-Linked Dominant Pattern

affected fathers have affected daughters; unaffected mothers have unaffected sons

Sex-Linked Recessive Pattern

unaffected fathers have unaffected daughters; affected mothers have affected sons

Chromosome Map

shows the amount of distance between

two genes that are located on the same chromosome

Closely Linked Genes

recombination and crossing over will occur less often

Distantly Linked Genes

recombination and crossing over will occur more often

Recombination Frequency

number of times crossing over occurred compared to the total number of offspring

Chi-Square Test

determines "goodness of fit" of experimental data; if data "fits the expectation," the number calculated will be low

Critical Value

highest number calculated by Chi-Square that is permitted to support a hypothesis

Degrees of Freedom

(number of groups tested in an experiment) - 1

Epigenetics

occurs when the environment influences the activity and expression of genes

DNA Methylation

decreases the expression of genes in the affected DNA region

Histone Modification

causes condensation or compacting of the DNA to prevent gene expression