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Sci9ence of Heredity
Lessons 1 & 2
Lessons 1&2- Genetics
 Heredity
 Passing of traits from parent to offspring
 Alleles
 Different forms of a trait that a gene may have
 Ex. Having dimples or not having dimples?
 Genetics- study of heredity.
 The study of how
traits are inherited
through the
interactions of alleles
What are Phenotypes and Genotypes?
Phenotype: PHYSICAL appearance, or visible traits
Genotype: Organism’s GENETIC makeup, or alleles.
Homozygous: Two Identical alleles for a trait.
Heterozygous: Two Different alleles for a trait.
Mendel- The Father of Genetics
 Australian Monk
 Conducted genetic experiments using the garden pea.
 1st person to trace one trait through several generations.
 Mendel crossed 2 plants with different expressions of the
trait and found that the new plants all looked like one of
the two parents.
 Hybrids

They received different genetic information (alleles) for a trait
from each parent.
 Dominant
 Factors that dominate, or covered up.
 In peas – Tall Dominated over Short Ex. Tt
(heterozygous for tallness)
 Recessive
 Factors that seem to disappear
 In peas- the short formed disappeared
Which Alleles are dominant?
Which alleles are recessive?
Punnett Square
 Shows you all the ways in which alleles can
combined.
 Handy tool used to predict results.
 A lower case letter = recessive allele.
 An upper case letter = dominant allele.
 The letters form a code!!!
 Genotype
 Genetic makeup, of an organism

Ex. BB
 Phenotype
 The way an organism looks and behaves
 Physical appearance of an organism

Ex. Brown hair
 Most cells in your body have two alleles for every trait.
 Located on chromosomes.
 Homozygous
 An organism with two alleles that are the same
 Ex. Homozygous dominant (TT) or
 Homozygous recessive (tt)
 Heterozygous
 An organism that has two different alleles for a trait.
 Ex. (Tt) or (Rr)
Let’s Make a Punnett Square
 Punnet Square- a genetic tool that uses letters to
represent dominant and recessive alleles.
 letters representing two alleles from one parent are written
along the top.
 * Letters representing two alleles from the other parent are
placed down the side.
 Each square of the grid is filled in with one allele donated by
each parent.
Letters= possible genotypes
 Probability- helps determine the chance that
something will occur.
Male: top
(Ff)
(F) dominant for freckles
Female: side
(Ff)
(f) not having freckles
F
f
1
2
3
4
F
f
Male: top
(Ff)
(F) dominant for freckles
Female: side
(Ff)
(f) not having freckles
F
1
2
FF
F
3
Ff
4
Ff
f
f
ff
Child in Box 1 = homozygous for freckles
Phenotypic Ratio: 3:1
Which box represents a child who does not have
freckles: Box 4- ff
Box 3 represents: Ff- heterozygous for freckles
Using Pedigrees
 Pedigree
 Tool for tracing a trait through generations of a family.
 Important for breeding animals or plants
 When geneticists understand how a trait is inherited, they
can predict the probability that a baby will be born with a
specific trait.