Study Guide for Genetics Test
Gregor Mendel - The father of genetics, who studied inheritance in the late 1800s
Heredity- The passing of traits from parents to offspring
Genetics- The scientific study of inheritance or heredity
Gene – A part of a chromosome that codes for a trait
Alleles - An allele is one of two or more versions of a gene. An individual inherits
two alleles for each gene, one from each parent. They are usually represented
with a letter and combine to form the genotype for each gene.
Homozygous - The two alleles are the same, RR (pure dominant) or rr (recessive)
Heterozygous or Hybrid- a genotype that is a combination of a dominant and a
recessive allele – Rr – Dd
Dominant gene- Strong form of a gene, which is expressed even if a recessive
gene is present – represented by a capital letter in the genotype (RR, Rr)
Recessive gene- The weak form of a gene, which is not expressed when the
dominant gene is also present – only expressed when both genes are recessive –
represented by a lower case letter in the genotype (rr)
Phenotype- A genetic trait that an individual actually shows, (photo) – the words
that describe the trait. Eg. Purple flower – Blue eyes – Short arms
Genotype- The gene combination that determines the phenotype. Eg. Kk – Rr - VV
Incomplete Dominance- A condition that results when genes produce a trait
somewhere in between the traits of the parents , skin color on a mixed race child.
Somatic cell - is almost any cell forming the body of an organism other than a
gamete. Human body cells have 46 chromosomes
Gametes- Sex cells; egg and sperm in organisms that reproduce sexually. Human
sex cells have 23 chromosomes or 1 from each of the parent chromosome pairs.
Egg cells – female sex cell. All of the 400,000 egg cells a woman will ever produce
are already present in her ovaries when she is born
Sperm cells – male sex cell, In Humans formed through meiosis beginning at
puberty and continuing for the rest of the male’s life.
Study Guide for Genetics Test
Mitosis – Cell division that produces 2 daughter cells identical to the 1 parent cell
– occurs for growth and to replace dead cells
Meiosis- Cell division that produces gametes (sex cells: eggs and sperm) or spores
having one set of unpaired chromosomes – 1 cell creates 4 gametes using Meiosis
Chromosomes – The normal human cell contains 46 chromosomes organized in
23 pairs.
Karotype – a picture of a person’s chromosomes
X chromosome-The longer sex chromosome; females have two X chromosomes,
Y chromosome-The shorter sex chromosome, males have one X and one Y
chromosome
Mutation- A sudden, unexpected change in the structure of the DNA or the
chromosomes; mutations can occur in body or sex cells; most are not harmful
Replication- The copying process in which new DNA is created
DNA Replication - adenine (A) always is paired with thymine (T) and cytosine (C)
pairs with guanine (G)
-- Punnant Square

For example, let's say that for the red-throated bird , red throat is the dominant trait and
white throat is recessive.
Since the "red-throat code" and the" white-throat code" are alleles (two forms of the same
gene), we abbreviate them with two forms of the same letter. So we use "R" for the
dominant allele/trait (red throat) and "r" for the recessive allele/trait (white throat).
R
r
r
R
 
R
R
r
Rr
Rr
r
Rr
Rr
One more note: A very very helpful thing to memorize is that the ONLY way for a recessive trait
to show up in an organism is if that organism's genotype is recessive (two little letters, like "rr").
Study Guide for Genetics Test