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Embryology and Heredity

A&P 2

Terminology list:

· Fertilization

· Zygote

· Blastocyst

· Embryo

· Fetus

· Chorion

· Amnion

· Placenta

· Allele

· Phenotype

· Genotype

· Autosomal dominant

· Autosomal recessive

· Sex-linked trait

Stages of development and events:

· Pre-embryo stages

· Secondary oocyte

· Fertilized ovum

· 1-cell zygote

· 2-cell zygote

· 4-cell zygote

· 8-cell zygote

· Morula

· Blastocyst

· Ovulation

· Fertilization

· Fusion of pronuclei

· Cleavage

· Differentiation

· Hatching

Prenatal development beginning with fertilization and ending in birth, approximate 38 weeks later, is divided into three periods:

__________ period: Week 1-2

Developmental changes from a single cell (zygote) to implantation of a multicellular sphere, the blastocyst

__________ period: weeks 3-8

Formation of extraembryonic membranes, germ layers, and organs

__________ period, weeks 9-38/birth

Maturation of tissues and organs, and rapid growth of the body

The stage at which the developing pre-embryo enters the uterus is the __________.

When the blastocyst hatches, the __________ __________ comes off.

What is the event that accomplishes pregnancy?

About how long does it take for the blastocyst to implant?

The __________ period begins when all organ systems have been established at about 9 weeks.

Extraembryonic membranes protect the embryo and assist with important functions such as nutrition, gas exchange, and removal of wastes.

List the structures that develop from each of the extraembryonic membranes:

· Amnion

· Chorion

· Allantois

· Yolk sac

The placenta is made of two components:

The fetal portion of the placenta is called the __________

The maternal portion of the placenta is called the __________ __________ which is composed of endometrium.

Primary Germ Layers:

Color-code the germ layers as follows:

· Ectoderm: Blue

· Mesoderm: Red

· Endoderm: Yellow

Name the primary germ layer that formed each of the structures listed below:

Epidermis of the skin _______________________________________________________

Heart _____________________________________________________________________

Liver _____________________________________________________________________

Bone _____________________________________________________________________

Hair ______________________________________________________________________

The following image depicts early embryonic events. In the following questions, identify the events, cell types, or processes referring to the figure.

1. Event A:

2. Event B:

3. Cell resulting from Event B:

4. Event C:

5. Embryonic structure C1:

6. Event D:

7. Embryonic structure at D:

8. Event E:

Trace the events of development from fertilization to implantation. In your tracing, you will do two things: (1) describe the changes that the conceptus undergoes from the time that fertilization takes place until the time that it implants; and (2) trace the conceptus’ location as these events occur. Trace the pathway using the image, and also fill in the space provided.

Start: Sperm and secondary oocyte meet in uterine tube _______________________________________

______________________________________________________________________________________________________

______________________________________________________________________________________________________

______________________________________________________________________________________________________

______________________________________________________________________________________________________

______________________________________________________________________________________________________

_________________________________________________________________________ End: Implanted blastocyst

Heredity:

GENETICS TERMINOLOGY

1.
Gene — a unit of heredity on a chromosome

1.
Allele — alternate forms of a gene (one allele comes from mom and one allele comes from dad)

1.
Locus — the location of a gene on a chromosome

1.
Dominant allele — alleles that mask the expression of other alleles but are themselves expressed (indicated with a capital letter)

1.
Recessive allele — alleles whose expression is masked by dominant alleles (indicated with a lower case letter)

Example: Dimples are dominant over none. We would use a capital
D for dimples present and a lower case
d for none. Alleles for the same gene must be represented by the same letter…capital for the dominant allele and lower case for the recessive allele.

1.
Homozygous — both alleles are the same (both dominant or both recessive)

1.
Heterozygous — alleles are different (one is dominant, one is recessive)

Example: Dimples are dominant over none. (D = dimples; d = none)

· If both alleles are dominant, the organism is said to be

homozygous dominant

. (DD)

· If both alleles are recessive, the organism is said to be

homozygous recessive

. (dd)

· If one allele is dominant and one is recessive, the organism is said to be

heterozygous.
(Dd)

Identify the following as homozygous dominant, homozygous recessive, or heterozygous:

6. Ww

6. bb

6. TT

6. Rr

6. GG

1.
Genotype — The genetic formula or statement of what alleles are present (there are three genotypes: homozygous dominant, homozygous recessive, or heterozygous)

1.
Phenotype — The physical expression of inherited traits; what the organism looks like; what you see when you look at the organism

Write the genotype in letter form for the following. Use the letters P and p.

8. Homozygous recessive

8. Homozygous dominant

8. Heterozygous

For each of the following, write the genotype. Before you start, identify each allele. Red is dominant over white. Red =
; white =

1. Homozygous red

1. Heterozygous red

1. White

Identify the following as a genotype or a phenotype.

1. BB

1. Purple flowers

1. heterozygous

1. Has freckles

COMPLETE DOMINANCE (SIMPLE DOMINANCE)

· There are only 2 possible phenotypes.

· Only 1 dominant allele is required for the trait to be expressed.

· 2 recessive alleles must be present for the recessive trait to be expressed.

Problem:

Dimples are dominant over no dimples. If a homozygous individual with dimples is crossed (mated) with a homozygous individual with no dimples, what will be the genotypes and phenotypes of the offspring?

Step 1 – Identify alleles: Dimples =
_________ No dimples =
(what letters are you going to use?)

Step 2 – Write out the cross:
x
(This is the
parental, or P1, generation)

Step 3 – Set up a Punnett square: arrange alleles along the side and top and show all possible combinations in the Punnett square.

This first generation of offspring is called the

first filial, or F1, generation.

Step 4 Identify genotypic and phenotypic ratios:

Each F1 individual can produce 2 different gametes, P and p. Gametes from these individuals combine with each other randomly.

Now cross two of your F1 gametes from your Punnett square above.

x

What is the genotypic ratio?

DD :
Dd :
dd

What is the phenotypic ratio?

Dimples :
no dimples

Example Problems:

Freckles are dominant to no freckles. Mom is heterozygous for freckles, and dad does not have freckles. Work the genetics problem. Identify the alleles, show the cross, show the Punnett square, and identify the genotypic and phenotypic ratios.

Dimples are dominant over no dimples. A man homozygous for dimples marries a woman with dimples whose mother lacked dimples. What proportion of their children will have dimples? Identify the alleles, show the cross, show the Punnett square, and answer the question.

INCOMPLETE DOMINANCE

Some traits will show a 3rd
intermediate phenotype if the genotype is heterozygous.

Problem:

Wavy hair in humans is a result of incomplete dominance between straight hair (H) and curly hair (h). If a man with curly hair marries a woman with wavy hair, what kinds of hair could their children have? Identify the alleles, show the cross, show the Punnett square, and show the genotypic and phenotypic ratios.

Identify alleles:

Curly hair =

Wavy hair=

Straight Hair =

Show the cross:
x

Fill in the Punnett Square.

Genotypic ratio:
HH
Hh
hh

Phenotypic ration:
curly
wavy
straight

LETHAL INHERITANCE

Lethal inheritance involves inheriting a gene that kills the offspring. Huntington’s disease is an autosomal dominant disorder caused by a mutation in the HTT gene and is an example of a lethal inheritance. An individual who has one copy of the mutated gene (Hh) will develop the disease, while an individual with two normal copies (hh) will not.

A man (Hh) with Huntington’s disease marries a woman (hh) who does not have the disease.

1. What are the possible genotypes of their children?

2. What is the probability that their children will inherit Huntington’s disease?

CODOMINANCE

Codominance occurs when both alleles contribute to the heterozygous genotype. A good example is BLOOD TYPE.

The alleles A and B are
codominant.

Allele A is dominant over O.

Allele B is dominant over O.

Allele O is recessive.

There are 4 blood types— Type A

Type B Type AB Type O

Phenotype (blood type)	A	B	AB	O
Possible genotypes

Blood typing is often used to determine the identity of an individual in forensic work and paternity suits.

Problem:
A woman with type O blood is suing a man for child support. The man has type AB blood, and the child has type O blood. Does she have a case?

x

Suppose that a person having type B blood is married to someone having type A blood. What are the possible blood types of their children?

A child with type O blood is born to a mother with type A blood.

What is the genotype of the child? ___________

What are the possible genotypes of the father? ___________________________________

Show a Punnett square to back up your answers.

SEX LINKED INHERITANCE

There are two kinds of sex chromosomes in humans, X and Y. If you are a male, you have one of each, or XY. If you are female, you have the XX genotype.

· Sex linked traits are

recessive
.

· Most alleles for sex linked traits are carried on the X chromosome, not the Y.

· Therefore, sex linked traits like hemophilia and red-green colorblindness are more commonly seen in males or females?

Problem:

A man with normal vision marries a woman with normal vision (her father was colorblind).

What is the chance that they will have a colorblind child? ________________________%

What is the chance that they will have a colorblind daughter? _____________________%

What is the chance that they will have a colorblind son?
_______________%

x

Hemophilia is a sex linked disorder that affects the blood’s ability to clot. Ronnie and Susan decide to get married. Susan has hemophilia. Ronnie does not. They are concerned about passing the disorder to their children. Work the genetics problem and identify the alleles, show the cross, show the Punnett square, and answer the following questions:
a. If the couple has a son, what is the chance that he will have hemophilia?
b. If the couple has a daughter, what is the chance that she will have hemophilia?

So far, everything we have done has represented the

monohybrid cross
, a genetic cross involving only one trait. When we work with two traits at the same time, it is called a

dihybrid cross
.

Example of a dihybrid cross:

In corn, a purple (R) seed color is dominant to yellow (r), and smooth seeds (S) are dominant to wrinkled (s) seeds. Consider the following cross:

RRSS x rrss.

· What is the phenotype of RRSS?

· What is the phenotype of rrss?

· What is the expected genotype for the F1 generation?

· Will all of the offspring have the same genotype?

· What is the phenotype of the F1 generation?

· Cross two F1 plants. Write the cross here:
x

· Do a Punnett Square:

How many different genotypes are possible in the F2 generation?

Consider the phenotypes of the F2 generation. How many are:

Red and smooth?

Red and wrinkled?

White and smooth?

White and wrinkled?

USING DNA BIOTECHNOLOGY TO DETECT GENETIC DISORDERS:

Sickle Cell Disease

Normal red blood cells are biconcave discs; people with sickle cell disease have red blood cells that are sickle shaped.
Draw some normal blood cells and then some sickle shaped blood cells:

Normal shaped cells Sickle shaped cells

Red blood cells pass through narrow capillaries carrying oxygen and carbon dioxide to and from body tissues. The sickle shaped cells cannot pass through these narrow passages. They tend to clog vessels and break down, causing poor circulation, anemia, and poor resistance to infection. Further complications can include jaundice, pain in abdomen and joints, and damage to internal organs.

The difference between normal red blood cells and sickle shaped cells is caused by a difference in

! This is an example of a genetic mutation.

Gel Electrophoresis:

Figure 11.1 Gel Electrophoresis of hemoglobin

Sickle cell hemoglobin (HbS) migrates slower toward the positive pole than normal hemoglobin (HbA) because the amino acid valine has no polar R groups. The amino acid glutamate does have a polar R group.

Detection by genomic sequencing:

Pretend you are a genetic counselor. A young couple comes to you for counsel because sickle-cell disease occurs among the family members of each. You order DNA base sequencing to be done. The results come back that at one of the alleles for normal hemoglobin, each has the abnormal sequence of CAC instead of CTC. The other allele is normal.

What are the chances that this couple will have a child with normal red blood cells?

_% What are the chances that this couple will have a child with sickle shaped cells?
%

What are the chances that this couple will have a child with normal cells, but is a carrier for sickle cell disease?
%