Terms to know in Mendelian Genetics
Mendel's law of dominance
There is always a pair of factors ( genes, alleles) which controls each character in an individual. One whose expression is observed in hybrid is dominant while another is recessive.
Mendel's law of segregation
Alleles segregate from one another during the formation of gametes.
Mendel's law of independent assortment of alleles.
Alleles of different genes are assorted independently of one another during the formation of gametes.
alleles
The different forms of a gene. Y and y are different alleles of the gene that determines seed color. Alleles occupy the same locus, or position, on chromosomes.
Autosomal
A locus on any chromosome but a sex chromosome. Not sex-linked.
Co-dominant alleles
Both alleles express their phenotypic effect when present in a heterozygote. For example, the alleles A and B are said to be co-dominant in case of human blood groups.
Complete linkage.
Complete linkage describes the inheritance patterns for 2 genes on the same chromosome when their is no crossing over.
Dioecious
Unisexual; Organisms produce only one type of gamete; i.e. humans
Dominant trait.
A trait expressed preferentially over another trait.
Drosophila melanogaster
The fruit fly, a favorite organism for genetic analysis.
Epistasis.
One gene masks the expression of a different gene for a different trait.
F1 generation
Offspring of a cross between true breeding plants, homozygous for the trait of interest
F2 generation
Offspring of a cross involving the F1 generation.
Genotype
The genetic makeup of an organism with respect to a trait. For a single trait either YY or Yy.
Hemizygous
If there is only one copy of a gene for a particular trait In a diploid organism, the organism is hemizygous for the trait, and will display a recessive phenotype. X-linked genes in fly or human males are hemizygous.
Heterozygous
Differing alleles for a trait in an individual, such as Yy.
Homologous chromosomes
The pair of chromosomes in a diploid individual that have the same overall genetic content. One member of each homologous pair of chromosomes in inherited from each parent.
Homozygous
Both alleles for a trait are the same in an individual. They can be homozygous dominant (YY), or homozygous recessive (yy).
Hybrid
heterozygous; usually referring to the offspring of two true-breeding (homozygous) individuals differing in the traits of interest.
Incomplete dominance
Intermediate phenotype in F1, parental phenotypes reappear in F2. The flowers of the snapdragon plant can be red, pink, or white. Color is determined at a single locus. The genotype RR results in red flowers and rr results in white flowers. The heterozygote genotype of Rr results in pink flowers. When the heterozygote has a different, intermediate phenotype compared to the homozygous dominant or homozygous recessive individuals, this is said to be incomplete dominance.
Lethal alleles.
Mutated genes that are capable of causing death.
Linkage.
genes that are inherited together on the same chromosome. Three inheritance patterns are possible: non-linkage, Partial linkage, and complete linkage.
Monoecious
Bisexual; Organisms produce both male and female gametes; i.e. garden pea.
Monohybrid cross.
Cross involving study of only one character.
Mutation
Sudden change in the DNA sequence of a gene to some new, heritable form. Generally, but now always a recessive allele.
Phenotype
The physical appearance of an organism with respect to a trait, i.e. yellow (Y) or green (y) seeds in garden peas. The dominant trait is normally represented with a capital letter, and the recessive trait with the same lower case letter.
Recessive trait.
The opposite of dominant. A trait that is preferentially masked.
Reciprocal cross
Using male and female gametes for two different traits, alternating the source of gametes.
Sex chromosomes
Sex determination is based on sex chromosomes
Sex-linked.
A gene coded on a sex chromosome, such as the X-chromosome linked genes of flies and man.
Back cross
Cross of an offspring with any of the parent.
Test cross
Generally a cross involving a homozygous recessive individual. When a single trait is being studies, a test cross is a cross between an individual with the dominant phenotype but of unknown genotype (homozygous or heterozygous) with a homozygous recessive individual. If the unknown is heterozygous, then approximately 50% of the offspring should display the recessive phenotype.
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