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Lecture 6

NATS 1675 Lecture Notes - Lecture 6: Punnett Square, Dihybrid Cross, Abo Blood Group System

Natural Science
Course Code
NATS 1675
Robert Crippen

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NATS 1675
- GENETICS TERMINOLOGY 2016-02-24 20:28 - GENETICS TERMINOLOGY 2016-02-24 20:28
1. Species is defined as populations of organisms that retain their individuality in nature
because they are reproductively isolated from other species around them. Reproductive
isolation includes both behaviour and genetics.
- Can include distance or barriers
- Genes have to pair up to form a functional organism
- Related species mate, they can and do produce offspring
- Hybrid: An organism produced form 2 different species or genotypes
- Example:
o Female horse + male donkey = Mule
o Male horse (64) + female donkey (62) = Hinny
o Changing the order of parents can greatly impact the result
of the offspring
- Hybrids are usually incapable of reproduction themselves
- Chromosomes: They are simple enough that the parents can reproduce, but different
enough that the hybrids offspring can not reproduce
2. Genotype—The genetic makeup of an organism, e.g., RR, Rr or rr; the combination of
alleles at a particular locus, or all loci.
- Genes usually work in pairs
- RR: Homozygous or round
- Rr: Homozygous, has one of each gene (Hybrid)
- rr: Recessive
3. Phenotype—The appearance of an organism with regard to an inherited characteristic (trait)
or all inherited characteristics. The phenotype is produced by the genotype in cooperation
with the environment,
- Example:
o R = Round pea seeds,
o r = wrinkled pea seeds.
o (R = Dominant, r = recessive)
4. Locus (loci pl.)—Location of a gene on a chromosome. Locus is sometimes used
synonymously for ‘gene’.
Mule + Hinny =
63 Chromosomes

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5. Allele—A pair of genes that occupy corresponding positions on homologous chromosomes.
(Remember—it is homologous chromosomes that pair during prophase I).
- Alleles are also 1 of 2 or more alternate forms of a gene that occupy a particular locus on
homologous chromosomes.
- ABO blood types are an example of 3 alternate forms of a gene found at a one locus
within the population.
6. Dominant—A trait that appears in the phenotype whenever the allele (gene) for it is present
in the genotype, e.g. RR or Rr = Round seeds.
7. Recessive—A trait that appears in the phenotype only when 2 alleles for the trait are present
in the genotype, e.g., rr = wrinkled seeds.
8. Homozygous—Alleles of a gene on homologous chromosomes are alike. [Remember—it is
homologous chromosomes that pair during Prophase I.]
Gene for Eye
Gene for Eye
Gene in the same location
-One will be dominant over the other
Homologous chromosomes (Pair
during Prophase I)

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9. Heterozygous—Alleles of a gene on homologous chromosomes are not alike.
- Homo = Same
- Hetero = Different
- Homozygous dominant or homozygous recessive
- Blue eyes = homozygous recessive
- Heterozygous cannot be recessive or dominant
10. Carrier—A heterozygous individual (organism) of normal phenotype who has a hidden
recessive allele in its genotype and thus can transmit the recessive allele to offspring. ‘Carrier’
and ‘heterozygous’ are synonyms, however, the term ‘carrier’ can imply a possibly damaging or
unwanted gene.
G1—The ‘Father of Genetics’ is Gregor Mendel, an Austrian monk, who presented his
research in 1865. Mendel’s paper contained 3 major generalizations:
o (1) The Principle of Dominance,
o (2) The Principle of Segregation, and
o (3) The Principle of Independent Assortment or Recombination.
G2—1) Dominance—When parents differ in one characteristic, their offspring (which are
hybrids) resemble one or the other parent, not a blend.
G3—2) Segregation—When a hybrid (e.g., offspring from the above parents) reproduces, its
reproductive cells are of 2 kinds—half transmitting the dominant character of 1 of its parents,
and the other half transmitting the recessive character of its second parent. [Alleles for any
given trait in homologous chromosomes separate (segregate) into separate gametes.]
Synapsis – Paring of homologous chromosomes
G4—3) Independent Assortment or Recombination—When parents differ in 2 or more
pairs of characters, each pair shows dominance and segregation independently of the other
Applies to the inheritance of 2 or more traits.
Genetic information for each trait is distributed to gametes independently of the distribution
of the other traits.
[Refers to the random separation or segregation of 2 or more pairs of alleles as long as they
are on different chromosomes.]
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