Chapter 14-15 | The Chromosomal Basis of Inheritance

Mendel’s Law of Segregation

Mendel's first law, stating that the two alleles in a pair segregate (separate from each other) into different gametes during gamete formation

Mendel’s Law of Independent Assortment

Mendel's second law, stating that each pair of alleles segregates, or assorts, independently of each other pair during gamete formation; applies when genes for two characters are located on different pairs of homologous chromosomes or when they are far enough apart on the same chromosome to behave as though they are on different chromosomes

Wild type (W+)

The phenotype most commonly observed in natural populations; also refers to the individual with that phenotype

Sex-linked trait

A gene located on either sex chromosome. Most sex linked genes are on the X chromosome and show distinctive patterns of inheritance; there are very few genes on the Y chromosome

Parental type

An offspring whose phenotype that matches one of the true-breeding parental (P generation) phenotypes; also refers to the phenotype itself

Recombinant

An offspring whose phenotype differs from that of the true-breeding P generation parents; also refers to the phenotype itself

Linkage map

A genetic map based on the frequencies of recombination between markers during crossing over of homologous chromosomes

One map unit

A unit of measurement of the distance between genes. Is equivalent to a 1% recombination frequency

Nondisjunction

An 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. Can lead to trisomy or monosomy

Aneuploidy

A chromosomal aberration in which one or more chromosomes are present in extra copies or are deficient in number

Polyploidy

A chromosomal alteration in which the organism possesses more than two complete chromosome sets. It is the result of an accident of cell division

Genomic imprinting

A process that regulates gene expression by silencing one copy of a gene from either the mother or father. Expression of the allele in offspring depends on whether the allele is inherited from the male or female parent

Hemizygous

Having only one copy of a gene or chromosome segment, instead of the usual two

Thomas Morgan’s fruit fly experiment

Demonstrated that traits like eye color are inherited through chromosomes. In his study, red eye color (dominant, W+) was crossed with white eye color (recessive). The F1 generation showed all red eyes, and the F2 generation showed a 3:1 ratio, with white eyes appearing only in male offspring. This established that genes on the X chromosome follow a sex-linked inheritance pattern, confirming that chromosomes are the basic unit of inheritance

SRY (sex-determining region on the Y)

A gene that is responsible for development of the testes in an embryo

Karyotype

A display of the chromosome pairs of a cell arranged by size and shape

X-inactivation

A process that occurs in female mammals during embryonic development. It inactivates one of the two X chromosomes in each cell, ensuring that the correct number of genes are expressed. This process balances the number of X-linked genes between females and males

Autosome

A chromosome that is not directly involved in determining sex; not a sex chromosome

Dependent assortment

Where different traits are inherited together, meaning the alleles for different genes would not separate independently during gamete formation, essentially staying linked and passing on specific combinations of alleles as a unit

Crossing over

The reciprocal exchange of genetic material between nonsister chromatids during prophase I of meiosis

Recombination frequency

( # of recombinants / total # of offspring ) x 100%

Maximum Recombination Frequency

Multiple crossover events during meiosis can result in a 50/50 split of recombinants and parentals for genes that are very far apart. This makes genes appear to assort independently, even though they are physically linked on the same chromosome

Autosomal Aneuploids

Trisomy 21 (Down Syndrome), Trisomy 13

Aneuploids on sex chromosomes

• XO - Monosomy X - female is sterile

• XXX - Trisomy X - female is completely normal

• XXY - Klinefelter - males that develop female characteristics

• XYY - Jacobs syndrome - causes increased height and learning or behavioral issues

Deletion

A piece of the chromosome is removed/missing

Duplication

A piece of DNA is repeated

Inversion

Reversing the order of a piece of the chromosome

Translocation

Removing a segment of DNA from a chromosome and adding it to another piece

Epistasis

A type of gene interaction in which the phenotypic expression of one gene alters that of another independently inherited gene

Codominance

The situation in which the phenotypes of both alleles are exhibited in the heterozygote because both alleles affect the phenotype in separate, distinguishable ways

Pleiotropy

When a single gene or DNA variant affects multiple traits in an organism

Polygenic inheritance

Occurs when one characteristic is controlled by two or more genes

Quantitative trait

A heritable feature that varies continuously over a range rather than in an either-or fashion

Multifactorial traits

Referring to a phenotypic character that is influenced by multiple genes and environmental factors

Trisomy

Referring to a diploid cell that has three copies of a particular chromosome instead of the normal two

Monosomy

Referring to a diploid cell that has only one copy of a particular chromosome instead of the normal two

Endosymbiont theory

The theory that mitochondria and plastids originated as prokaryotic cells engulfed by a host cell. The engulfed cell and its host cell then evolved into a single organism

Mitochondrial Inheritance

The genetic pattern where individuals inherit their mitochondrial DNA solely from their mother, meaning that only females pass on their mitochondrial DNA to their offspring. This is considered "maternal inheritance" because the mitochondria are exclusively inherited from the mother's egg cell

Chromosomal Aberrations

Gene duplication, inversion, deletion, and translocation