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Expression of Blood Types
Blood type Genotype Pattern of inheritance
A I A I A or I A i I A is dominant to i
B I B I B or I B i I B is dominant to i
AB I A I B I A is co-dominant to I B
O ii Two recessive alleles

Certain combinations of alleles can be lethal, meaning they prevent the individual from developing in utero, or cause a shortened life span. In recessive lethal    inheritance patterns, a child who is born to two heterozygous (carrier) parents and who inherited the faulty allele from both would not survive. An example of this is Tay–Sachs, a fatal disorder of the nervous system. In this disorder, parents with one copy of the allele for the disorder are carriers. If they both transmit their abnormal allele, their offspring will develop the disease and will die in childhood, usually before age 5.

Dominant lethal inheritance patterns are much more rare because neither heterozygotes nor homozygotes survive. Of course, dominant lethal alleles that arise naturally through mutation and cause miscarriages or stillbirths are never transmitted to subsequent generations. However, some dominant lethal alleles, such as the allele for Huntington’s disease, cause a shortened life span but may not be identified until after the person reaches reproductive age and has children. Huntington’s disease causes irreversible nerve cell degeneration and death in 100 percent of affected individuals, but it may not be expressed until the individual reaches middle age. In this way, dominant lethal alleles can be maintained in the human population. Individuals with a family history of Huntington’s disease are typically offered genetic counseling, which can help them decide whether or not they wish to be tested for the faulty gene.

Mutations

A mutation    is a change in the sequence of DNA nucleotides that may or may not affect a person’s phenotype. Mutations can arise spontaneously from errors during DNA replication, or they can result from environmental insults such as radiation, certain viruses, or exposure to tobacco smoke or other toxic chemicals. Because genes encode for the assembly of proteins, a mutation in the nucleotide sequence of a gene can change amino acid sequence and, consequently, a protein’s structure and function. Spontaneous mutations occurring during meiosis are thought to account for many spontaneous abortions (miscarriages).

Chromosomal disorders

Sometimes a genetic disease is not caused by a mutation in a gene, but by the presence of an incorrect number of chromosomes. For example, Down syndrome is caused by having three copies of chromosome 21. This is known as trisomy 21. The most common cause of trisomy 21 is chromosomal nondisjunction during meiosis. The frequency of nondisjunction events appears to increase with age, so the frequency of bearing a child with Down syndrome increases in women over 36. The age of the father matters less because nondisjunction is much less likely to occur in a sperm than in an egg.

Whereas Down syndrome is caused by having three copies of a chromosome, Turner syndrome is caused by having just one copy of the X chromosome. This is known as monosomy. The affected child is always female. Women with Turner syndrome are sterile because their sexual organs do not mature.

Career connections

Genetic counselor

Given the intricate orchestration of gene expression, cell migration, and cell differentiation during prenatal development, it is amazing that the vast majority of newborns are healthy and free of major birth defects. When a woman over 35 is pregnant or intends to become pregnant, or her partner is over 55, or if there is a family history of a genetic disorder, she and her partner may want to speak to a genetic counselor to discuss the likelihood that their child may be affected by a genetic or chromosomal disorder. A genetic counselor can interpret a couple’s family history and estimate the risks to their future offspring.

For many genetic diseases, a DNA test can determine whether a person is a carrier. For instance, carrier status for Fragile X, an X-linked disorder associated with mental retardation, or for cystic fibrosis can be determined with a simple blood draw to obtain DNA for testing. A genetic counselor can educate a couple about the implications of such a test and help them decide whether to undergo testing. For chromosomal disorders, the available testing options include a blood test, amniocentesis (in which amniotic fluid is tested), and chorionic villus sampling (in which tissue from the placenta is tested). Each of these has advantages and drawbacks. A genetic counselor can also help a couple cope with the news that either one or both partners is a carrier of a genetic illness, or that their unborn child has been diagnosed with a chromosomal disorder or other birth defect.

To become a genetic counselor, one needs to complete a 4-year undergraduate program and then obtain a Master of Science in Genetic Counseling from an accredited university. Board certification is attained after passing examinations by the American Board of Genetic Counseling. Genetic counselors are essential professionals in many branches of medicine, but there is a particular demand for preconception and prenatal genetic counselors.

Visit the National Society of Genetic Counselors website for more information about genetic counselors.

Visit the American Board of Genetic Counselors, Inc., website for more information about genetic counselors.

Chapter review

There are two aspects to a person’s genetic makeup. Their genotype refers to the genetic makeup of the chromosomes found in all their cells and the alleles that are passed down from their parents. Their phenotype is the expression of that genotype, based on the interaction of the paired alleles, as well as how environmental conditions affect that expression.

Working with pea plants, Mendel discovered that the factors that account for different traits in parents are discretely transmitted to offspring in pairs, one from each parent. He articulated the principles of random segregation and independent assortment to account for the inheritance patterns he observed. Mendel’s factors are genes, with differing variants being referred to as alleles and those alleles being dominant or recessive in expression. Each parent passes one allele for every gene on to offspring, and offspring are equally likely to inherit any combination of allele pairs. When Mendel crossed heterozygous individuals, he repeatedly found a 3:1 dominant–recessive ratio. He correctly postulated that the expression of the recessive trait was masked in heterozygotes but would resurface in their offspring in a predictable manner.

Human genetics focuses on identifying different alleles and understanding how they express themselves. Medical researchers are especially interested in the identification of inheritance patterns for genetic disorders, which provides the means to estimate the risk that a given couple’s offspring will inherit a genetic disease or disorder. Patterns of inheritance in humans include autosomal dominance and recessiveness, X-linked dominance and recessiveness, incomplete dominance, codominance, and lethality. A change in the nucleotide sequence of DNA, which may or may not manifest in a phenotype, is called a mutation.

Questions & Answers

what is the difference between a neuron and nerve?
Tonny Reply
Neurons are specialized cells which are capable of transmitting signals between different parts of the body. Nerve is a bundle of fibers composed of neurons. Therefore, neurons and nerves are closely related.
acquosuah
wow
Andy
what are the first aid procedures
Andy
immediately care to prevent further injury
Vandana
what is a positive and negative feedback and give examples of both positive and negative feedback
esther Reply
Positive feedback is a feedback that tends to magnify its output. An example of positve feedback is the release of oxytocin from the pitutary gland during child birth. Negative feedback regulates a stimulus to cause a opposite effect. An example of this is when you eat your blood sugar rises,which
Camello
is sensed by the nervous system. specialized cells in the pancreas sense the increase and release the hormone insulin.
Camello
merci beaucoup
esther
what then is Pathological anatomy
acquosuah Reply
anatomy study about normal human organ... pathological anatomy study of diseased organ
Aswin
thanks
acquosuah
wow
Morshed
that's are good
Morshed
why oral glucose make insulin response in 1 phase but IV glucose make insulin response in 2 phase?
qwe Reply
what is anatomical position?
Kakande Reply
it's de description of any region or part of the body in specific strance
Andy
the standard anatomical position is standing straight with your hand an feet forward and the neck straight and face facing forward
Lubabah
how many cell do we have in our body
Sawmtei Reply
around 37.2 trillion
Jeremiah
how do you know when a women is lying
Hernandez
go with your gut
Stephanie
what is a positive and negative feedback and give examples of negative and positive feedback
esther Reply
messa I didn't understand this too
Shammy
and would love to know as well
Shammy
what are the various types of white blood cells
Andy Reply
what is cpr in first aid
Andy
various whiteblood cells includes Granulocytes (neutrophils,basophils ana eosinophils) and Agranulocytes (monocytes and lymphocytes)
Waziri
Cardiopulmonary resuscitation (cpr) is an emergency procedure. Uses chest compressions with artificial ventilation in an effort to preserve brain function until further measures are taken to restore blood circulation and breathing in a person who is in cardiac arrest.
Jeremiah
wow thanks alot
Andy
respiration is not just taking in of oxygen and bringing out Co2. that is called INHALATION AND EXHALATION. BUT RESPIRATION IS THE BREAKDOWN OF LARGE MOLECULES OF GLUCOSE OR OTHER SUBSTRATES IN THE PRESENCE OF OXYGEN AND SUBSEQUENT REMOVAL OF WASTE PRODUCT
Osuji Reply
why study anatomy?
esther Reply
to know the detailed functioning of the internay body organs
Jam
we study anatomy to know about the structure of the organs which in turn help to study physiology which is the knowledge related to function of the vital organ,and when we know the normal functionality we can understand the abnormalities in that organ,and the study of disease is called pathology
Lubabah
why is it so hard to know the spelling and words
Sawmtei
they were made so that only the best and thr brightest would understand.
Senen
thank you
esther
to know the internal structure of the human body and how it function
Jaafar
whats antonmy
Jaan Reply
the study of structure and function of internal body parts
Jam
is a branch of biology which describes the structures of the body and relationship from one part to another
esther
I believe it is the scientific study of body structures that are both macro and micro.
acquosuah
wat the question is tis ! without knowing tis simple thing.. y didn't install tis app ..
Aswin
what is anatomy and physiology
mwitwa Reply
Anatomy is the study of structures of the body
Zunehri
Physiology the study functions of the body
Zunehri
anatomy is study of structure of body nd physiology is study of function of body.
PRIYANKA
anatomy is the study of internal body structures and physiology is the functioning of these structures in the body
Jam
what is lungs
Shipon Reply
it an organ found in our thoracic cavity
Richard
lungs are spongy organs where our respiration takesplace
Maryama
Lungs is a spongy organs located in the chest , is an a primary organs of respiration in human body and other Animals
Zunehri
lungs thorex me present hote hai ye organ hai and gas exchange (o2,co2 exchange)inka inka function hai.
PRIYANKA
lungs ka function hai
OM
pls am a student I don't know
Mavis
main function is respiration
faheem
respiration is the taking ing of oxygen by the body tissues and the removal of carbondioxide from the body tissues
Jam
what is heart beat,?
Naqeeb Reply
heart beat is the process briting
Mavis
what functions of heart
Muthu Reply
To circulate the blood
Arshad
it pump blood to the lungs
Laura
very twiest
Shipon

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Source:  OpenStax, Anatomy & Physiology. OpenStax CNX. Feb 04, 2016 Download for free at http://legacy.cnx.org/content/col11496/1.8
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