Unit 4 Genetics Part 1
Heredity
M.Elizabeth
Martin Luther King, Jr. JHS
2006
Chapter 6 Heredity
6.1 Mendel and His Peas
6.2 Meiosis
Chapter 6 Review
6.1 Mendel and His Peas
Heredity – the passing of traits from parents
to offspring;
Gregor Mendel
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Grew up on a farm – why he was familiar with
nature
He became a priest and studied at the
University
Observant and curious – what if?? Why is it
so?
Chose to study pea plants
How Do you Like Your Peas
Garden sweet peas were chosen by
Mendel because:
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Grow quickly
Usually self pollinating
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Contain both male and female
reproductive structures
Fertilization can take place within the
same flower
Pistil
Anther
Filament
Sepal
Stamen
Petal
Stigma
Style
Ovary
Ovule
Pollination – Sexual Reproduction
There are two types of reproduction:
Asexual – a single parent produces an
identical offspring.
Lots of organisms reproduce this
way (bacteria and algae). When
cells are cloned they are reproduced
in this way.
Sexual – requires two parents. Results
in a lot better chance for genetic
variation.
Pollination Process
Pollen containing sperm from the
anthers (male) is transferred to the
stigma (female).
 Fertilization occurs when a sperm
from the pollen travels through the
stigma and enters the egg in an ovule.
The transmission of
characteristics/traits from one
generation to another is called
heredity
____________
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Peas be My Podner – Controlled Experiments
Mendel chose to study only one
characteristics (trait) at a time:
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Plant height
Seed shape
Flower color
Seed color
Pod color
Pod shape
Flower position
Controlled Experiments
True Breeding Plants – When these plants
self pollinate they always produce
offspring with the same trait that the
parent plant has (homozygous)
Mendel crossed two plants that had
different forms of a single trait using
cross-pollination.
Cross Pollination – the anthers of one
plant are removed so that the plant
cannot self-pollinate
Then pollen from another plant is used to
fertilize the plant without anthers.
Mendel’s First Experiment
Mendel performed crosses to study seven
different characteristics (traits)
Each trait was separately crossed:
Example: the cross between round
seeds and wrinkled seeds plants
produce offspring called the first
generation (F1)
Mendel found that one trait always
appeared and the other trait vanish???
The trait that appeared – dominant
The trait that disappeared - recessive
Mendel’s Second Experiment
First generation plants cross by self
pollination
 Mendel performed this experiment and
found for the two traits studied that
the recessive trait shows up again
 Mendel got to counting the second
generation (F2) to figure out what was
going on and calculated ratios to
determine the dominant to recessive
ratios.
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Genes and Alleles
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Mendel realized that there must be two
sets of instructions for each characteristic
and that each parent would donate one set
of instructions to offspring
Now we know that the sets of instructions
are genes and that a fertilized egg would
have two forms of the same gene for every
characteristics and we call these two forms
alleles.
Mendel’s Scientific Fame
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Mendel’s ideas were not immediately
accepted by the scientific community when
he published his results in 1865,
It was more than 30 years later that his
discoveries were recognized as significant.
Once Mendel’s ideas were discovered in
part by William Batson and Reginald Punnett
the door to modern genetics was opened.
Punnett Square
To understand how Gregor Mendel
came to his conclusions, a Punnett
Square is used.
 Punnett Square – a tool used to
visualize all the possible combinations
of alleles from the parents.
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How to make a Punnett Square
1.
2.
3.
4.
Draw a square and divide it into four
sections
Write the letters that represent alleles
from one parent along the top of the box
Write the letters that represent alleles
from the other parent along the side of
the box
To make the cross bring one allele down
and one over so that each of the four
squares has now 2 alleles showing the
genotypes that are possible with the
cross
Probability
The mathematical chance that an
event will occur – likelihood
 Probability is usually expressed as a
fraction (ratio) or a percent.
 To express probability as a
percentage, divide the fraction and
multiply by 100.
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1
in 2 = ½ = 0.5 that when multiplied by
100 is 50%.
6.2 Meiosis
Two types of reproduction: sexual and
asexual.
 Asexual
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 only
one parent cells is needed
 The parent cells copies it’s internal
structures (mitosis) and then divides
producing new cells that are exact copies
of the parent cell
Sexual Reproduction
Two parents are needed
 Each parent produces a sex cell (egg or
sperm) which are different from
ordinary cells.
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 Ordinary
human cells contain 46
chromosomes (23 pairs). Each pair is
called homologous chromosomes.
 Sex cells contain 23 chromosomes (half
the usual number.
Less is More – Meiosis to the Rescue
Because each sex cell has one-half the
number of chromosomes when each sex
cell is united during fertilization, the
resulting zygote has a both halves of
the number of chromosomes.
 Meiosis is the process whereby sex
cells are produced.
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Meiosis