Genetics Part I: Introduction
Nestor T. Hilvano, M.D., M.P.H.
(Images Copyright Discover Biology, 5th ed., Singh-Cundy and Cain,
Textbook, 2012.)
Learning Objectives
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3.
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6.
Define key terminologies.
- alleles, dominant, recessive, homozygous,
heterozygous, genotype, phenotype, P generation, F1
generation, and F2 generation
Give the translation of the following word stems.
– co-, di-, pleio-, poly-,gen-, and gametDefine monohybrid cross and illustrate an example.
Explain how Mendel’s law of segregation describes the
inheritance of a single characteristic.
Explain how Mendel’s law of independent assortment
applies to dihybrid cross. Illustrate an example of
dihybrid cross.
Discuss how a testcross is performed to determine the
genotype of an organism.
Wordstems
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•
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co- together
di- two
pleio-more
poly- many
gen- produce
gamet- wife or husband
Science of Genetics
• Blending hypothesis – characteristics from both parents
blend in the offspring (like blue and yellow paint blend to make
green)
• Particulate hypothesis - is the idea that parents pass on
discrete heritable units (genes)
- Gregor Mendel documented this through experiments
with garden peas; Father of modern genetics
• Genetics:
- Trait (physical appearance specified by genes);
Ex: eye color, shape of thumb, height
- Types of Trait (set of genetic trait for each form);
Ex: Eye color= brown eyes(B) and blue eyes (b);
Thumb’s shape= straight thumb (H) and hitchhiker’s thumb (h)
- Heritable trait= passed from parent to offspring
Mendel’s Genetics
• Self-fertilization developed truebreeding varieties
• Cross-fertilization between 2
different plants produced hybrids
(genetic cross)
• Hypothesized that there are
alternative forms of genes for
specific trait= Alleles
• Each of us has 2 alleles- ex: (BB,
bb, Bb);(HH, hh, Hh)
• P generation= True-breeding
(parents)
• F1 generation= hybrid offspring
• F2 generation= product of F1
cross
• Monohybrid cross-1 trait (ex: flower
color: purple and white)
• Dihybrid cross- 2 traits (flower color:
purple and white, and seed color: yellow
and green)
Monohybrid Cross: Law of Segregation
• Pair of genes segregate during formation of
gametes; the genes pairs once again at
fertilization
- Purple (PP)= (P) (P), white (pp)= (p) (p)
• Homozygous genotype - identical alleles;
Ex: PP (homozygous dominant); pp
(homozygous recessive)
• Heterozygous genotype - two different
alleles (Ex: Pp)
• Dominant allele (trait)- capital letter;
determines appearance; ex: (P)
• Recessive allele (trait) -lower case; no
noticeable effect on appearance; ex: (p)
• Phenotype – expression of trait or
appearance; ex: purple, white
• Genotype – actual genetic make-up; ex:
PP, Pp, or pp
• Punnett square – shows the possible
combinations of alleles (genetic cross)
♂ possible gametes
♀ possible
gametes
• Monohybrid cross (1 trait) Ex: Flower
color, P (purple); p (white)
• Possible combinations: PP (purple); Pp
(purple); pp (white)
• Phenotype ratio: 3:1 (3 purple:1 white)
• Genotype ratio: 1:2:1
(1PP:2Pp:1pp)
Monohybrid Cross
• Ex. 1 Trait – eye color of fruitfly
dominant trait – Red (R)
recessive trait – white (r)
• P generation:
Red (RR) x white (rr)
• F1 generation? All _______________
• F2 generation? __________________
• Fill in punnett square (diagram for predicting the results
of genetic cross).
♂ possible gametes
♀ possible gametes
Genotype ratio? ___________________
Phenotype ratio? __________________
Law of Independent Assortment
• Dihybrid cross: one pair segregate
independently from the second
pair during gamete formation
(independent assortment)
• F1 produce 4 gamete genotypes
• F2 generation:
– 9/16 had round yellow seeds
– 3/16 had wrinkled yellow seeds
– 3/16 had round green seeds
– 1/16 had wrinkled green seeds
• Genotype ratio: 1:2:1:2:4:2:1:2:1
• Phenotype ratio: 9:3:3:1
Dihybrid Cross
• Ex. 2 traits – eye color and straight hair
dominant traits – Brown eye (B)
- Straight hair (S)
recessive traits – blue eye (b)
- curly hair (s)
P generation: BBSS x bbss
F1= BbSs x BbSs
F1 produce 4 genotypes: BS, Bs, bS, bs
F2=Phenotype ratio: 9:3:3:1
Give the phenotype of: BS= _____; Bs= _____;
bS= _____; bs= _____
Testcross
• Use of the test is to identify the genotypes
responsible for the phenotypes of the progenies
after a cross
• Crossing of unknown genotype (expressing
dominant phenotype) and a homozygous
recessive individual
• Ex. Tall plant(T_) and dwarf plant(tt)
2 possibilities for Tall plant: TT or Tt
• Do a cross of each possibility.
t
T
T
t
T
t
t
Homework
1.
2.
3.
Define terms: genotype; phenotype; alleles;
monohybrid cross; dihybrid cross; test cross; law of
segregation
Differentiate homozygous alleles and heterozygous
alleles, and dominant trait and recessive trait.
The allele for straight hair (S) is dominant to the allele
for curly hair (s). A father with straight hair who is
heterozygous (Ss) has 4 children with his wife who is
also heterozygous (Ss). Construct a punnett square
showing the location of gametes and offspring. Identify
offspring as to genotype ratio and phenotype ratio.
What is the phenotypic trait (expressed appearance) of
child with ss alleles? with Ss alleles?