Dominant vs. Recessive Alleles

Genetics Week 2

I Can…

I can define the terms gene and allele.

I can describe the difference between dominant and recessive alleles.

I can explain the difference between phenotype and genetype.

I can define codominance.

Vocabulary

Gene Allele Dominant Allele Recessive Allele Heterozygous Homozygous Genotype Phenotype

What are genes and alleles? We call the factors that control traits GENESGENES.

We call the different forms of a gene ALLELESALLELES.

For example, the gene for pea plant flower color comes in two alleles: purple or white.

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Back to Mendel…

Mendel wanted to know how it was possible for a trait like white pea plant color to disappear in one generation and then reappear in the next.

His conclusion: the trait is always there--its just hidden by another trait!!

Dominant and Recessive Alleles Mendel came up with the terms DOMINANT and RECESSIVE alleles to describe how some traits could be “hidden” or “masked” by other traits.

Dominant and Recessive Alleles DOMINANT ALLELE: the trait that always shows up in the organism whenever it is present.

RECESSIVE ALLELE: the trait that is masked, or covered up, whenever the dominant allele is present.

DOMINANT

RECESSIVE

How do we represent alleles?

We use a capital

letter for dominant alleles

We use a lower case letter

for recessive alleles

D = dominant trait

d = recessive trait

How many alleles do we have for each trait?

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Organisms inherit Organisms inherit

22 alleles for alleles for each trait.each trait.

Organisms inherit Organisms inherit

22 alleles for alleles for each trait.each trait.

DdDdDdDd

How do these alleles work?

How do these alleles work? A trait controlled by a recessive allele will ONLY “show up” if the organism inherits two recessive alleles for the trait.

What trait will the baby have? In humans, brown eye color is dominant (B) and blue eye color (b) is recessive.

If the baby gets:– BB, it gets the DOMINANT TRAIT

– Bb, it gets the DOMINANT TRAIT

– bb, it gets the RECESSIVE TRAIT

Sponge Bob Genetics

Under the sea, yellow body color is dominant to blue. What color body would the following babies end up with?

YY _________________ Yy _________________ yy _________________

Homozygous vs. Heterozygous

We call someone with two

IDENTICAL alleles for a trait

HOMOZYGOUS for that trait.

We call someone with two

DIFFERENT alleles for a trait

HETEROZYGOUS for that trait.

BBBB bbbb BbBb bBbB

GENOTYPE: the

inherited combination of alleles

(the LETTERS)

Ex: Dd, BB

PHENOTYPE: the inherited appearance

(the physical traits you

Ex: Dimples

Genotype vs. Phenotype It is impossible to determine a person’s GENOTYPE just by looking at them if they have the dominant PHENOTYPE.

For example, brown eyes could be the result of BB or Bb

Am I BB or

BBBB BbBb

Genotype vs. Phenotype

When a person shows a recessive phenotype, however, we can always tell their genotype just by looking at them.

For example, the ONLY way to get blue eyes is by having a bb genotype.

I know my genotype is bb for eye color.

Why do Dominant Traits show up more than Recessive Traits?

This is because there are 3 combinations of alleles that make a DOMINANT TRAIT. (BB, Bb, bb)

There is only 1 allele combination that makes a RECESSIVE TRAIT (bb)

Sponge Bob Genetics

For each genotype below, indicate whether it is a heterozygous (He) OR homozygous (Ho).

TT _____ Bb _____ DD _____ Ff _____

Which of the genotypes in #1 would be considered purebred? Which of the genotypes in #1 would be hybrids?

Determine the phenotype for each genotype using the information about SpongeBob.

Square is dominant to round. SS ___ Ss ____ ss ___

For each phenotype, give the genotypes that are possible for Patrick.

Tall head (T) is dominant to short (t). Tall = __Short = __

Human Reproduction and Alleles

Genetics Week 2

I Can…

I can explain that sexual reproduction produces offspring that inherit half their DNA from each parent.

I can describe the difference between a diploid and a haploid cell and explain that human sex cells have 23 chromosomes.

Human Chromosomes Within the nucleus of almost every cell in the human body there are 46 chromosomes.

These chromosomes come in pairs.

Human Chromosomes

So…humans have 23 pairs of chromosomes in every cell’s nucleus.

We say that cells with the full set of 23 pairs or 46 total chromosomes are DIPLOID.

Chromosomes and Life It is extremely important to have all 46 chromosomes.

When an individual is missing a chromosome or has an extra chromosome serious sometimes life threatening complications occur.

Chromosomes and Life These complications arise because the genes we need to survive are carried on these chromosomes.

Genes are just sections of the DNA that make up chromosomes.

Chromosomes and Reproduction So, how do humans get the full set of 46 chromosomes?

Chromosomes and Reproduction

Because humans reproduce through sexual reproduction, they inherit half or 50% of their DNA from each parent.

So, they inherit 23 chromosomes from each parent.

Inheriting Chromosomes

But if human cells have 46 chromosomes each, how does the baby not end up with too many chromosomes?

46 from Mom

46 from Dad

Sex Cells Sex cells, the sperm and the egg, are special cells because each of these cells contains only 23 chromosomes.

When egg and sperm meet the 23 chromosomes match up and make 23 chromosome pairs or 46 total chromosomes.

Mom’s Dad’s

Sex Cells Because each sex cell has only 23 chromosomes we say they are HAPLOID (think “HAlf-diPLOID”

46/2= 23

What will the baby look like?

Because each chromosome carries genes, when the two sex cells meet and the chromosomes form pairs, the baby ends up with 2 alleles for each trait.– One allele is carried on the Dad’s chromosome.

– One allele is carried on the Mom’s chromosome.

Meiosis

Genetics Week 2

How do sex cells end up with just 23 chromosomes?

MEIOSIS: is the process by which sex cells are made. It results in cells with half the usual amount of chromosomes.

Meiosis vs. Mitosis

Remember: MITOSIS produces cells with the same amount of DNA. MEIOSIS produces sex cells with half of the DNA.

Beginning of Meiosis

Before meiosis begins, every chromosome in the cell is copied.

Meiosis I

The chromosome pairs line up next to each other in the center of the cell.

The pair then separate from each other and move to opposite ends of the cell.

Two cells form each with half the number of chromosomes.

Homologous Chromosomes = Chromosome Pairs

Meiosis II The chromosomes move to the center of the cell. The centromeres split and the chromatids separate.

(This is basically just mitosis for each of the cells!)

End of Meiosis

Four sex cells have been produced from the original cell.

Each sex cell has half the number of chromosomes that the parent cell had.

Each cell has only one chromatid from each original pair.

These will become eggs or

sperm.

What does this mean for alleles?

Since alleles are carried on chromosomes, when the chromosomes are seperated in meiosis, one allele from the parent cell goes to each egg or sperm cell.

What does this mean for alleles?

When the egg and sperm fuse, the new baby ends up with an allele pair for every gene.

Dominant vs. Recessive Alleles

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