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Random segregation

This diagram shows the genetics experiment conducted by Mendel. The top panel shows the offspring from first generation cross and the bottom panel shows the offspring from the second generation cross.
In the formation of gametes, it is equally likely that either one of a pair alleles from one parent will be passed on to the offspring. This figure follows the possible combinations of alleles through two generations following a first-generation cross of homozygous dominant and homozygous recessive parents. The recessive phenotype, which is masked in the second generation, has a 1 in 4, or 25 percent, chance of reappearing in the third generation.

Mendel’s observation of pea plants also included many crosses that involved multiple traits, which prompted him to formulate the principle of independent assortment. The law states that the members of one pair of genes (alleles) from a parent will sort independently from other pairs of genes during the formation of gametes. Applied to pea plants, that means that the alleles associated with the different traits of the plant, such as color, height, or seed type, will sort independently of one another. This holds true except when two alleles happen to be located close to one other on the same chromosome. Independent assortment provides for a great degree of diversity in offspring.

Mendelian genetics represent the fundamentals of inheritance, but there are two important qualifiers to consider when applying Mendel’s findings to inheritance studies in humans. First, as we’ve already noted, not all genes are inherited in a dominant–recessive pattern. Although all diploid individuals have two alleles for every gene, allele pairs may interact to create several types of inheritance patterns, including incomplete dominance and codominance.

Secondly, Mendel performed his studies using thousands of pea plants. He was able to identify a 3:1 phenotypic ratio in second-generation offspring because his large sample size overcame the influence of variability resulting from chance. In contrast, no human couple has ever had thousands of children. If we know that a man and woman are both heterozygous for a recessive genetic disorder, we would predict that one in every four of their children would be affected by the disease. In real life, however, the influence of chance could change that ratio significantly. For example, if a man and a woman are both heterozygous for cystic fibrosis, a recessive genetic disorder that is expressed only when the individual has two defective alleles, we would expect one in four of their children to have cystic fibrosis. However, it is entirely possible for them to have seven children, none of whom is affected, or for them to have two children, both of whom are affected. For each individual child, the presence or absence of a single gene disorder depends on which alleles that child inherits from his or her parents.

Autosomal dominant inheritance

In the case of cystic fibrosis, the disorder is recessive to the normal phenotype. However, a genetic abnormality may be dominant to the normal phenotype. When the dominant allele is located on one of the 22 pairs of autosomes (non-sex chromosomes), we refer to its inheritance pattern as autosomal dominant    . An example of an autosomal dominant disorder is neurofibromatosis type I, a disease that induces tumor formation within the nervous system that leads to skin and skeletal deformities. Consider a couple in which one parent is heterozygous for this disorder (and who therefore has neurofibromatosis), Nn , and one parent is homozygous for the normal gene, nn . The heterozygous parent would have a 50 percent chance of passing the dominant allele for this disorder to his or her offspring, and the homozygous parent would always pass the normal allele. Therefore, four possible offspring genotypes are equally likely to occur: Nn , Nn , nn , and nn . That is, every child of this couple would have a 50 percent chance of inheriting neurofibromatosis. This inheritance pattern is shown in [link] , in a form called a Punnett square    , named after its creator, the British geneticist Reginald Punnett.

Questions & Answers

What is the skin
Astony Reply
What is physiology?
Njaleny Reply
it is the study of the structure of the human body and its reationship
Hannah
Physiology is the function of a living organism.
Tammy
It is the study of human body and how it works.
Excellent
It is d entrance to the uterus
Umunna Reply
It is a small sensitive part of female sex organ which is found above d entrance to the vagina
Umunna
A small hole in d skin which contains d root of a hair
Umunna
A thin membrane which covers d entrance to the vagina in young girls
Umunna
Is a gland that produces milk in women and other female mammals
Umunna
A repeated process in which a woman's uterus gets ready for pregnancy which ends in a period if she does not get pregnant
Umunna
hello
Tamba
hi sir
Mir
so in summary describe the level of structural complexity within the body
Freda Reply
how can we describe briefly the levels of organization
Frimpong
what is blood clotting
Yahya Reply
Tiny bits in your blood called platelets get "turned on" by triggers released when a blood vessel is damaged. They stick to the walls in the area and each other, changing shape to form a plug that fills in the broken part to stop blood from leaking out. When activated, platelets also release chemic
Nilesh
what is a cell membrane?
lameck Reply
the cell membrane also called the plasma membrane regulates the transport materials entering and existing the cell.
Frimpong
what's gonna happen if your body doesn't produce enough white blood cells?
Jovanna
how is speech controlled by the nervous system?
Joseph Reply
The broca's area
Udebuana
what does the HCG
Njaleny
chemical level cellular level tissue level organ level organ system level organism
Chidera Reply
Yes Yes because they can perform more than one activity in the body
Chidera
the functional and structural unit of the body
Chidera
A group of 2 or more tissue that come together to perform a specific function
Chidera
A group of organs that work together to perform a specific function or meet the physiological needs of the body
Chidera
It's the highest level of organization
Chidera
2 or more cells coming together to perform a specific function
Chidera
Skeletal muscles can be tetanized but not cardiac muscle.Discuss
Okeke Reply
what is regional anatomy?
Grace Reply
study of structures that contribute to specific body region
charles
what is human anatomy?
Sam
It's the study of the interrelationships of body structures in a specific region
TM
is the study of the structure of the body and the relationship between body systems
Frimpong
what is grey matter
OJO Reply
it is the major component of the central nervous system consisting of the neuronal cell bodies,neuropil,glial cells,capillaries and synapses
Frimpong
What is neuron cell
Dev Reply
neuron cell are the fundamental units of the brain and nervous system responsible for receiving sensory input from the external world, for sending motor commands to muscles and for transforming and relaying the electrical signals at every step in between
Serphine
What is the perfect defination of Life
Dev
What does a word muscles means ?
Abubakar
the perfect destination of life is to have life without flaws and defects.Alife where nothing can go wrong.
Frimpong
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Hannah Reply
because she's not yet at term
Nansi Reply
what is physiology?
Grace Reply
what is physiology
David
Physiology is the study of the functions of the human body
Hannah
am a new one!!
sekela
what is Homeopathy
AHMED

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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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