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Dominance and Punnett Practice
Complete Dominance
Gray fur is dominant (G) to orange fur (g). Just like the
Punnett squares we have been practicing.
Incomplete Dominance
The two alleles “blend” together to create a combination
of the traits
Co-Dominance
Both alleles are shown in the phenotype. One is NOT
dominant over the other.
Sex Linked Traits
Because the gene controlling the trait is located on the sex chromosome,
It is linked to the gender of the individual. Usually found on the X chromosome.
The result is that females will have two copies of the sex-linked gene while males
will only have one copy of this gene. If the gene is recessive, then males only need
one recessive gene to have a sex-link trait.
Examples of Sex-linked Traits:
Red-green colorblindness
Male Pattern Baldness
Hemophilia
Duchenne Muscular Dystrophy
Dominant vs. Recessive
• Genotype= the alleles that you have for a trait
• Phenotype= the physical expression of those alleles
• A person with a dimpled chin can have the genotype DD
or Dd
• A person with a non-dimpled chin can only have the
genotype dd
More Definitions
•Homozygous= two of same allele
(DD or dd)
•Heterozygous= two different alleles
(Dd- also called hybrid)
Human Chromosomes
•46 total chromosomes, 23 pairs
•23rd pair = sex chromosomes (female = XX , male = XY)
• The other 22 pairs are called autosomes
• ALL egg cells carry a single X; sperm carry either an
X or a Y
• This means a woman can ONLY give an X chromosome, a man has a
50/50 chance of giving an X or a Y
Meiosis
• Meiosis is the process of making gametes (sperm and
eggs)
• Gametes have half the number (23) of chromosomes of
the original cells (46)
• Meiosis turns one 46 chromosome cell into four 23
chromosome cells
Meiosis: How it Happens
23
46
Chrom
46
Chrom
23
92
Chrom
Interphase
(Double
Chromosomes)
46
Chrom
23
Meiosis I
23
Meiosis II
After Meiosis
A sperm cell and egg cell, each with 23 chrom., fertilize to form a
zygote.
+
23
=
23
The cell of the zygote divides/grows to become an embryo, and
eventually a baby!
The zygote has 46 chromosomes!
Muy Importante!
Meiosis is necessary because if you merged two cells with 46
chromosomes each to make a baby, you would get a 92
chromosome mutant!
You must half the number of chromosomes to get sperm + egg = 46!
Why Meiosis? Evolutionary Advantage
• Sexual reproduction increases the genetic variety in
a population
• Greater variety = stronger population
• Sexual reproduction always involves the combining
of genes from two different organisms to form a
zygote
• Plants, animals, fungi, protists…
Asexual Reproduction
• Asexual reproduction also exists in simple
organisms, but makes a clone of the parent
• This DOES NOT increase genetic variety
• Some organisms do both types of reproduction
• Potatoes form sprouts that are
clones of themselves.
• They can also make
seeds/sperm to reproduce with
neighboring plants.