SG 4-2
Mendelian Genetics
Law of Segregation and the Testcross
 Gregor Mendel = “father of genetics”
- Worked with pea plant to study patterns of inheritance.
- His work disproved the hypothesis of “blending” to
explain concept of heredity.
o If this was true, all populations would end up the
same = no variation
- Explained inheritance – “particulate” hypothesis
o Parents pass on discrete heritable units (genes)
that have their separate identities in offspring.
- Called genes = (heritable) factors; no knowledge of
DNA
- Using peas, experimented with
o True breeding – offspring are just like parents
(Ex. Purple X Purple; White X White)
o Hybridization – crossed two different truebreeding pea varieties = offspring of true
breeders. (Ex. Purple X White)
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SG 4-2
 Mendel’s conclusions from his experimentation:
He was way before his time on this model. How did he come up
with all this without any knowledge of DNA/Chromosomes?
WOW, . . . what a brain!
1. There are alternate versions of factors (genes) that
contribute to a population’s variation.
 Alternate version of a gene= Allele
2. For each trait, an organism inherits two alleles; one from
each parent.
3. If the alleles are different:
- the organism will express the dominant allele
- the recessive allele will have no noticeable effect on
organism
4. Law of segregation = the two alleles for a gene will
segregate (separate) into different gametes during meiosis.
(Think Meta to Ana II !!)
- The use of Punnett squares, crossing one trait can
demonstrate this law.
 Remember these terms before we dive into Punnett squares?
 Homozygous = alleles are the same for a gene.
 Heterozygous = alleles are different for a gene.
 Genotype = actual genetic make-up (represented by letters;
of course we are not going to write out the genetic code for
each gene!)
 Phenotype = physical characteristic expressed
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SG 4-2
 In pea plants,
Yellow is dominant; green is recessive
Round is dominant; wrinkled is recessive
 Mendel’s “M.O.” 1. True breed cross: Cross purebred yellow with purebred
green = P generation (parent generation)
-note that all possibilities are (heterozygous) = F1 generation
(1st filial generation)
2. Monohybrid cross: Cross F1 X F1
- Results in F2 generation
- Genotypic ratio = 1:2:1
- Phenotypic ratio 3:1
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SG 4-2
 How can you figure out if the dominant phenotype is a result
of a heterozygous or homozygous dominant genotype? Do a
test cross!
3. Test cross
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