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Heredity & Mendelian Genetics
Gene – Segment of DNA that comprises a unit of heredity information. Genes exist as pairs of
alleles in diploid individuals.
Genome – Total set of genetic information in a specific organism.
Homologous chromosomes – Pairs of chromosomes similar in morphology (size, shape) and
genetic constitution. Humans have 23 pairs of homologous chromosomes, with one member
of a pair inherited from one parent, and one from the other. Example: Familiar sets of
homologous chromosomes include the “XX” sex chromosomes.
Locus (pl. loci) – Site on a chromosome at which the gene for a given trait occurs; Segment of
chromosomal DNA containing information that controls some feature of the organism. Ex:
Segment of DNA on a chromosome that controls plant height.
Alleles – Alternative forms of a specific gene that occupy corresponding loci on homologous
chromosomes. Ex: The different alleles for plant height can be represented by “T” for tall and “t” for
short.
Sexual reproduction – Entails the union of two gametes to form a single celled zygote. Introduces
genetic variation in offspring that allows adaptation to environmental changes.
Haploid (n) – Condition of having one member of each homologous pair. Ex: Common in sex cells of
sexually reproducing organisms.
Gametes – Haploid (n) sex cells formed by meiosis. Ex: Generally, male gametes are sperm and female
gametes are eggs.
Diploid (2n) – Condition of having two sets of homologous chromosomes per cell nucleus. The
diploid chromosome number for humans is 23 pairs of chromosomes for a total of 46. Ex:
Human body cells are diploid (except for gametes).
Zygote – Single diploid (2n) cell formed when two gametes fuse. Ex: In plants and animals, the zygote is
the fertilized egg.
Mitosis – Division of cell resulting in two genetically identical daughter cells with half the size of the
original cell. Mitosis duplicates genetic material thereby conserving chromosome number
and type.
Meiosis – Two successive divisions forming four haploid cells with ¼ the size as the original cell.
Meiosis reduces chromosome number by half (2nn) to maintain the appropriate number
of chromosomes for sexual reproduction. Results in gamete formation and introduces
genetic variation (recombination).
Homozygous – Having a pair of identical alleles for a particular characteristic or locus. Both alleles
are expressed. Ex: TT or tt.
Heterozygous – Having a pair of unlike alleles for a particular characteristic or locus. In most cases
only one of the alleles is expressed (dominant); Sometimes they interact. Ex: Tt
Dominant allele – An allele that is always expressed, regardless of whether the condition is
homozygous or heterozygous. Represented by a capital letter. Ex: The dominant allele is “T”, or
tall, in TT and Tt.
Recessive allele – Represented by a lower case letter. Ex: The recessive allele is “t” or short Tt and tt. This
allele is only expressed in a homozygous condition (“tt”).
Genotype – The entire genetic makeup of an individual. Actual gene composition. Ex: TT or Tt or tt.
Phenotype – Physical or chemical expression of an organism’s genes. Expression (what is observed)
of gene composition. Ex: TT (homozygous) and Tt (heterozygous) refer to tall plants, while tt
(homozygous) would indicate a short plant.
Document created by South Campus Library Learning Commons 7/29/06. Permission to copy and use is
granted for educational use provided this copyright label is displayed.
MENDEL’S LAWS OF GENETICS
Mendel was the first scientist to develop a method for determining the outcome of
inheritance patterns. He formulated several principles that are known as Mendel’s laws of
genetics.
Law of Dominance: When an organism has two different alleles for a trait, one
allele dominates.
Law of Segregation: During gamete formation by a diploid organism, the pair of
alleles for a particular trait separate, or segregate, during meiosis.
Law of Independent Assortment: The members of a gene pair separate from one
another independent of the members of other gene pairs. The separation occurs in
the formation of gametes during meiosis.
APPLICATION: Mendelian Crosses – Predicting the probability of inherited traits.
1.
A symbol is designated for each allele in the gene pair. A dominant trait is
represented by a capital letter, the recessive by a lower case. Since having a detached
earlobe is the dominant trait, use E for detached earlobes and e for attached.
2.
Determine the genotypes of the parents and the genotypes of the gametes. A
heterozygous male and a heterozygous female to be crossed have the genotypes of
Ee and Ee. During meiosis the allele pairs separate. A sperm cell contains either an
E or an e, while the egg cell also contains either an E or an e.
3.
Punnett Square – A Punnett square is a boxed figure used to determine the
probability of genotypes and phenotypes in the offspring of a genetic cross. The
possible gametes produced by the male are indicated on the left side of the square.
For heterozygous male Ee and heterozygous female Ee
Gametes from Female Eggs
e
E
Gametes from
Male Sperm
E
EE
Ee
e
Ee
ee
Document created by South Campus Library Learning Commons 7/29/06. Permission to copy and use is
granted for educational use provided this copyright label is displayed.
4.
From the Punnett square, the phenotype of each possible genotype can be
determined.
Genotypes:
EE
Ee
ee
Phenotypes:
Detached Earlobe
EE, Ee
Attached Earlobe
ee
Ratio of Phenotypes from Punnett Square:
3:1
Example:
Two Characteristics: Hair Color (Black B, brown b) and Eye Color (Brown R, blue r)
Parental Generation:
Homozygous Dominant
Homozygous Recessive
Black hair and Brown eyes
Brown hair and Blue eyes
↓
↓
BBRR
bbrr
F1 Generation: All heterozygous BbRr
Gametes formed by segregation and independent assortment of alleles → BR, Br, bR, br
Gametes from F1 Female
F2
BR
Br
bR
br
Generation
1
BR
BBRR
1
1
bR
1
br
BBRr
2
BBRr
Br
Gametes from
F1 Male
1
BbRR
BbRr
Key and Probabilities
1 – Black hair, brown eyes - 9/16
2 – Black hair, blue eyes – 3/16
3 – Brown hair, brown eyes – 3/16
4 – Brown hair, blue eyes – 1/16
1
1
BBrr
1
2
BbRR
BbRr
Bbrr
1
2
BbRr
3
3
BbRr
bbRR
bbRr
Bbrr
3
4
Ratio of Phenotypes:
bbRr
bbrr
9: 3: 3: 1
Document created by South Campus Library Learning Commons 7/29/06. Permission to copy and use is
granted for educational use provided this copyright label is displayed.