Transcript CH 6.3-6.5 Mendelian Genetics Class Notes

Inheritance of Traits:
An Introduction to Genetics &
Human Genetics
Chapters 6.3 - 6.5
1
Review
Which type of cells are responsible for passing traits
on from parent to offspring?
- Sex cells: sperm & egg
Which type of cell division makes these sex cells?
– Meiosis
What does this cell division do to chromosome #?
– Reduce chromosome # by half
What type of cells are made during mitosis?
- Body cells
2
What does this cell division do to chromosome #?
- Maintains the same/identical chromosome #
What part inside a nucleus carries ones traits?
– Chromosomes
What makes up a chromosome?
– DNA and histones
What is the study of how traits are passed from parents to
offspring?
- Genetics
What is the name of a scientist who studies Genetics?
- Geneticist
3
You have body cells and
Gametes
• The cells in your body are divided into two
groups.
– 1. Somatic cells – are your body cells that
make up your body.
– 2. Sex cells or gametes – the cells that make
up sperm and ova (egg).
• Gametes are responsible for passing on your DNA
to your offspring/children.
4
I. Chromosomes
A. Autosomes – also known as body cells
Two characteristics of autosomes:
• Chromosomes are paired. Always two of each
type of chromosome. One from mom and one
from dad.
• There are 22 pairs of autosomes.
B. Sex Chromosomes – Come from Gametes
Three characteristics of sex cells
• Half as many chromosomes
• Only one of each kind of chromosome
• Sperm or eggs
Karyotype
A picture which shows a
person’s chromosomes
6
There are thousands of genes on
a Chromosome
• Genes are small sections of chromosomes that
determine a persons traits (characteristics).
Eye color
• Genes are arranged one after another like beads on
a necklace
Eye color
Nose size
Widow’s peak
Shape of ears
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Genes come in pairs just like chromosomes.
One gene comes from mom, the other from dad.
Eye color
Nose size
Widow’s peak
Shape of ears
Eye color
Nose size
Widow’s peak
Shape of ears
 Every trait or characteristic has a gene pair to
represent it.
 Traits are inherited characteristics.
 Allele is an alternate form of a gene.
EXAMPLE: Gene – eye color
Alleles – blue, green, brown
8
Passing traits to offspring
Mom makes ova (egg) with her genes from her
chromosomes.
AND
F F
Dad makes sperm with his genes from his chromosomes.
•
f
f
The egg and the sperm unite through fertilization
• The zygote now gets one gene from the mother and
one from the father.
F
f
Ff
9
Gregor Mendel
• Was an Austrian monk who became known as the
Father of Genetics.
• Around 1868 he studied and worked with pea plants.
• Pea plants have 7 traits each with 2 contrasting alleles
– seed shape
– seed color
– plant height
– pod color
– pod shape
– seed coat color
– flower position
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Mendel’s 1st experiment
1. Used purebred (homozygous) plants
- This was known as the P generation (parents)
2. Control over breeding
- Mendel used pollen to fertilize selected pea plants.
Mendel controlled the
fertilization of his pea plants
by removing the male parts,
or stamens.
He then fertilized the female
part, or pistil, with pollen from
a different pea plant.
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Offspring were all purple.
12
Genes/Alleles
– Each parent donates
one allele for every
gene trait.
– Homozygous describes
two alleles that are the
same
– Heterozygous describes
two alleles that are
different
13
Mendel’s 1st Conclusion
• Law of Dominance
– Genotype = gene combination (represented by
alphabet letters)
– Phenotype = Physical appearance (represented by
words)
– A dominant allele is expressed as a phenotype
when at least one allele is dominant.
• Dominant = Capital letter (ex: T = tall)
• Dominant allele hides Recessive allele
– A recessive allele is expressed as a phenotype only
when two copies are present.
• recessive = Lowercase letter (ex: t =small) 14
Genotype- Phenotypegenes
appearance
Homozygous
Same 2 alleles
Heterozygous
Mixed Alleles
TT
Tall
tt
Short
Tt
Tall
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Dominant and Recessive Genes
(Homozygous = Having two of the same alleles)
(Heterozygous = Having two different alleles)
Homozygous Dominant: Have two of the same
(dominant) genes
Homozygous Recessive: Have two of the same
(recessive) genes
Heterozygous: Have one dominant and one
recessive gene (2 different)
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Mendel’s 2nd Conclusion
• Law of segregation
– Organisms donate only one copy of each gene in
their gametes.
– Thus, the 2 copies of each gene
segregate, or separate, during
gamete formation.
– Organisms inherit 2 copies of each gene, one from
each parent.
17
Traits and Probability
The Punnett square is a grid system for
predicting all possible genotypes & phenotypes
resulting from a cross.
– The axes represent
the possible gametes
of each parent.
– The boxes show the
possible genotypes
of the offspring.
– Shows ONLY the
probability for each
child (like rolling a die), it is chance!
18
Steps for using the Punnett square
in a Monohybrid Cross
Monohybrid Cross = a cross between only 1
trait or characteristic
1. Draw 4 boxes. Each square stands for
one possible combination of genes for
offspring.
Possible
offspring
Possible
offspring
Possible
offspring
Possible
offspring
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2. Draw eggs and write the letters for the
genes that will be in the Mother’s sex
cells on the top.
T
t
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3. Draw sperm and write the letters for the
genes that will be in the Father’s sex
cells on the side.
T
t
t
t
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4. Copy the letters that appear at the top of
the square into each box below it.
T
T
t
t
t
t
T
t
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5. Copy the letters at the left of the box into
the boxes next to each letter.
T
t
t
Tt
Tt
t
t t
t t
Look at all the
possible
combinations of
sperm and eggs.
These are the
possible
combinations of
genes an organism
can have.
23
Practice
• Do the punnett square for a heterozygous
couple for height
•
T
t
T
TT
Tt
t
Tt
tt
Mom Tt x Dad Tt
Complete:
Genotype
1: TT
2: Tt
1: tt
Phenotype
3:Tall (75%)
1: short (25%)
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Observed vs Expected
• The punnet square shows ONLY
the probability for each child– not for
the family as a whole. (like rolling a
die)
• The more data that’s observed, the
closer it gets to the EXPECTED.