Study Guide Answers Bio A

Genetics and Pedigrees

Identify the purpose and “set up” of a test cross.

• Purpose: Is a dominant phenotype is homozygous or heterozygous ?(Use with complete dominance)

• Setup: cross the dominant phenotype (Tt or TT (second allele is unknown) with a purebred recessive (tt).

• Offspring born with a recessive trait indicates a heterozygous genotype.

T ?

t

t

List and differentiate between the Law of Segregation and the Law of Independent assortment. What type

of chromosomes do each refer to?

• Law of Segregation refers to a pair of genes on homologous chromosomes (you don’t know which allele you will get). You get one or the other, but not both.

• Law of Independent assortment says that the random allele you get from any gene is independent of the allele you will get from another gene. If you get small height, that doesn’t mean you will get a tongue that curls. So the “set of chromosomes” (or genes) you get will be totally random. (not always true for alleles on the same chromosome).

How do current day molecular genetics support Mendel’s conclusions? Based on his laws, what

were Mendels three conclusions? Does present day molecular biology support his Laws? Explain which inheritance patterns support his claims and which

do not.

• Mendels conclusions are his three laws (dominance, segregation, independent assortment)

• His laws are true in the case of complete dominance and for genes located on different chromosomes (or far apart on the same chromosome)

(Continued) How do current day molecular genetics support Mendel’s conclusions? Based on his laws,

what were Mendels three conclusions? Does present day molecular biology support his Laws?

Explain which inheritance patterns support his claims and which do not

• His laws don’t hold up for co-dominance, incomplete dominance, sex-linked, polygenic, and multiple allele

• His laws don’t hold up for genes that are linked (found close together on the same chromosome)

What are three things we know now, that Mendel did not know?

What inheritance patterns do we now know?

• Alleles on genes can be A) codominant or B)incompletely dominant (more inheritance patterns than dominant)

• C) Some genes are on the same chromosome and can be linked.

• All inheritance not due to complete dominance patterns; we also have co-dominance, incomplete dominance, sex-linked, polygenic, and multiple allele

Explain why the Punnett square does not tell you what each of your four children would be but is

simply a visual that shows the random assortment and recombination of genes during meiosis and

fertilization

• Shows the likelihood of a genotype for any given child. What you actually get could be different.

• The alleles of the parents show the possible alleles for what the gamete (egg or sperm) will contain after meiosis. The combinations inside the box represent what the zygote will contain after fertilization.

• For each child, the same probabilities exist, each time a child is conceived.

Be able to do Punnett Square word problems

• Recognize: heterozygous, homozygous dominant, be able to identify the genotype of a phenotype,

• Be able to calculate percentages and ratios (labeling all of them).

• Know what the heterozygous will look like in codominance (both traits seen) and incomplete dominance (blending).

• Know that X linked means the male gets ONE allele.

Incomplete dominance example

• A true breeding tall black rose is crossed with a true breeding short black rose. Show a punnet square and identify the genotypes and phenotypes that would result (for height) if height was incompletely dominant

T T

t

t

Tt Tt

Tt Tt

All offspring would be medium and heterozygous

Co-dominance example

Assume a codominant inheritance pattern.

A blue and white flower is crossed with a pure blue flower.What is the probability of getting a blue flower?A white flower?

CB CW

CB

CB

CBCB CBCW

CBCB CBCW

50% blue, 0% white

Dihybrid test cross

• In the small ones only one for each trait. In a 16 square, it would show 2 (one for each trait)

• You combine the letters for each trait– SsTt or SSTT, the offspring would have four letters.– If you do test cross (and you have a heterozygous)

and cross with all recessive you would get 25% totally recessive for both characteristics

– If you do a test cross and you have a homozygous dominant, all offspring would be heterozygous.

Dihybrid test cross

• Create two test crosses, one for each characteristic. For each characteristic

T ?

t

t

T ?

t

t

Dihybrid Blue, Horned bird

• Mother : HhBb Horned, Blue• HB, Hb, hB, hb• Father : hhbb no horn, white– hb, hb, hb, hb

• OR Father HHBB Horn, Blue• HB, HB, HB, HB

HB Hb hB hb

HB

HB

HB

HB

CBCB CBCW

CBCB CBCW

HHBB

HHBB

HHBB

HHBB

HHBb

HHBb

HHBb

HHBb

HhBB

HhBB

HhBB

HhBB

HhBb

HhBb

HhBb

HhBb

HB Hb hB hb

CBCB CBCW

CBCB CBCW

hhbb

hhbb

hhbb

hhbb

HhBb

HhBb

HhBb

HhBb

Hhbb

Hhbb

Hhbb

Hhbb

hhBb

hhBb

hhBb

hhBb

hb

hb

hb

hb

What is a sex-linked trait?

• Any gene that is on either the X or Y chromosome is considered “sex linked”.

• A gene that is on only the X chromosome is X-linked. All genes that we have talked about that are on a sex chromosome are on the X

(hemophilia, colorblindness)

X linked show up more in males because:

• A male has only one X, while females have two.

• Since everything we studied on the X is a recessive disease, a female needs two XhXh and a male only needs one XhY allele to have the disease.

• A son can inherit the disease only from his mother. (he always gets the X from mom.

• A daughter who has the disease MUST have a father that has the disease, and a mother that carries the allele.

Linked genes

• Linked genes are genes that are on the same chromosome (this means whatever alleles are on that chromosome are transferred together when homologous chromosomes separate)

• When crossing over occurs, alleles are exchanged between the two homologous chromosomes. This means that alleles that were linked become “unlinked”.

When a pedigree shows an autosomal dominant disease, what key inheritance patterns do you see in

the pedigree?

• Doesn’t skip generations (no hidden alleles)

• A child MUST have a parent that has the disease

• If both parents have the disease, children CAN be born without the disease

Examples: Achondroplasia (Dwarfism)

Huntingdon’s disease

When a pedigree shows an X-linked disease, what key inheritance patterns do you see in the pedigree?

• If offspring have the disease, they are almost always males.

• A son has only ONE allele that dictates the disease. Since it is on the X chromosome it MUST come from his mother

• If a girl has the disease her father MUST have the disease.

• All carriers are female

Autosomal recessive

• Parents can both carry the allele, but not have the disease, but the offspring will inherit. Can be carried

• Since it is NOT on the X chromosome, both males and females get this disease equally.

• If both parents have the disease, ALL offspring will have the disease.

Cystic fibrosis

Tay Sachs

Sickle cell anemia (sort of )

Polygenic Characteristic

• A characteristic that ADDs together the effect of several genes In pigment, the more dominant alleles, the darker the skin. Distribution (normal curve, or continuous)

• Example: skin color, 3 genes (6 alleles). – No dominants albino–One dominant light skin, blonde hair– Five dominants dark brown eyes, skin, hair– Six dominant v dk br eyes, skin, hair black.

Polygenic

Normal distribution ORContinuous distribution OR Normal curve

Polygenic Characteristic

• A characteristic that ADDs together the effects of each dominant allele. Distribution normal curve or continuous)

• Example: height, 3 genes (6 alleles). – No dominants short (under 5 ft–One dominant 5’ – Five dominants 6’ 2– Six dominant Tall 6’ 5” or taller

Multiple allele characteristic

• More than two alleles for each gene , BUT, each person has only two of the possibilities

• Multiple allele is ONE gene with more than two alleles• Polygenic is more than one gene each with two alleles

• Example: Blood type alleles IA, IB, i• A and B are dominant to i (O) • A and B are codominant to each other

Some families have a “baldness” allele that is X-linked. Is it possible for a female offspring to show

this phenotype? Create a Punnett square that provides evidence to support your answer.

• If the father has the baldness gene XbY and the mother is a carrier XBXb, this can happen.

• But the both parents must have the gene and the father MUST have the disease (or the phenotype )..baldness is really not a disease.

• Research indicates that there are several kinds of baldness, and some types are

inherited from the father!!!!!