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By the end of this section, you will be able to:
  • Differentiate between genotype and phenotype
  • Describe how alleles determine a person’s traits
  • Summarize Mendel’s experiments and relate them to human genetics
  • Explain the inheritance of autosomal dominant and recessive and sex-linked genetic disorders

We have discussed the events that lead to the development of a newborn. But what makes each newborn unique? The answer lies, of course, in the DNA in the sperm and oocyte that combined to produce that first diploid cell, the human zygote.

From genotype to phenotype

Each human body cell has a full complement of DNA stored in 23 pairs of chromosomes. [link] shows the pairs in a systematic arrangement called a karyotype    . Among these is one pair of chromosomes, called the sex chromosomes    , that determines the sex of the individual (XX in females, XY in males). The remaining 22 chromosome pairs are called autosomal chromosomes . Each of these chromosomes carries hundreds or even thousands of genes, each of which codes for the assembly of a particular protein—that is, genes are “expressed” as proteins. An individual’s complete genetic makeup is referred to as his or her genotype    . The characteristics that the genes express, whether they are physical, behavioral, or biochemical, are a person’s phenotype    .

You inherit one chromosome in each pair—a full complement of 23—from each parent. This occurs when the sperm and oocyte combine at the moment of your conception. Homologous chromosomes—those that make up a complementary pair—have genes for the same characteristics in the same location on the chromosome. Because one copy of a gene, an allele    , is inherited from each parent, the alleles in these complementary pairs may vary. Take for example an allele that encodes for dimples. A child may inherit the allele encoding for dimples on the chromosome from the father and the allele that encodes for smooth skin (no dimples) on the chromosome from the mother.

Chromosomal complement of a male

This figure show the 23 pairs of chromosomes in a male human being.
Each pair of chromosomes contains hundreds to thousands of genes. The banding patterns are nearly identical for the two chromosomes within each pair, indicating the same organization of genes. As is visible in this karyotype, the only exception to this is the XY sex chromosome pair in males. (credit: National Human Genome Research Institute)

Although a person can have two identical alleles for a single gene (a homozygous    state), it is also possible for a person to have two different alleles (a heterozygous    state). The two alleles can interact in several different ways. The expression of an allele can be dominant, for which the activity of this gene will mask the expression of a nondominant, or recessive, allele. Sometimes dominance is complete; at other times, it is incomplete. In some cases, both alleles are expressed at the same time in a form of expression known as codominance.

In the simplest scenario, a single pair of genes will determine a single heritable characteristic. However, it is quite common for multiple genes to interact to confer a feature. For instance, eight or more genes—each with their own alleles—determine eye color in humans. Moreover, although any one person can only have two alleles corresponding to a given gene, more than two alleles commonly exist in a population. This phenomenon is called multiple alleles. For example, there are three different alleles that encode ABO blood type; these are designated I A , I B , and i.

Questions & Answers

what are the advantages of the concave shape of red blood cells?
Amy Reply
This structure is VERY flexible. It can allow these cells to get into the most tiny places in our bodies. a VERY good design! The advantage of red blood cells' biconcave shape is that the surface area is increased to allow more haemoglobin to be stored in the cell.
action of gluteus medius and minimus
Green Reply
Lateral rotation of the hip joint
Briefly explain location of ecg on a patient
Prince Reply
it is a machine that gives a graphical representation of heart beat
Briefly explain location of ecg leads on a patient?
in ecg we use electrical leads over the chest ,ancle and wrist
what is the anatomical and function difference between paravertebral and prevertebral ganglia ?
Rada Reply
types of tissue in human
Preety Reply
charactetistic Of cartilaginous tissue
what is theRecurrent infection?
pankaj Reply
what do you mean about recurrent infection
Recurrent or persistent infection is a manifestation of primary immuno deficiency
weakens the immune system, allowing infections and other health problems to occur more easily
lysis of RBC
What is barometric pressure
Kedha's Reply
what is the agglutination advantage
Gopal Reply
the functions of the liver
Nana Reply
it produces bile juice which is used to make the food smaller
it also plays an important role in conversion of amino acid into urea
it also has role in gluconeogenesis in which amino acids and lipids convert into glucose.
during fetal life it's a center for hemopoiesis (formation of blood cells)
it filters, or removes, harmful substances from the blood
It stores nutrients, such as vitamins and iron,for the body
what is the largest gland in human body
Shahid Reply
thyroid gland
thyroid is largest endocrine gland
describe microscopic structures of the kidney
Nana Reply
kidney is covered by fibrous capsule, consists of an outer cortex and inner medulla with medullary pyramids. The microscopic structure is seen as 1-2 millions of nephrons and collecting tubule.
identify the four major tissue types
Binkheir Reply
connective epithelial
two ramining
muscle nervous
epithelial, connective, muscle, and nervous tissue
tell me about urine formation
Nana Reply
it includes three steps. ultrafiltration selective reabsorption tubular secretion
ultrafiltration also known as glomerular filteration. All solutes up to 4nm size and water can freely pass through the filtering membrane.
selective reabsorption : About more than 99% of water ,electrolytes and other substances are reabsorbed by the tubular epithelial cells. The reabsorbed subtances move into interstitial fluid and then into blood of peritubular capillaries .
The substances like water ,glucose,amino acids and electrolyte are reabsorbed
tublar secretion: the substances are transported from blood to again into the renal tubules
and then those are excreted out as urine
internal and external structures of the kidney
Nana Reply
how the kidney works
on the bases of pressure and filtration
excretion of wasts, role in hb, role in vit D synthesis
care to explain?
yes plx
the differences between eukaryotic and prokaryotic
Binkheir Reply
eukaryotic don't have nucleus and prokaryotic have nucleus.
no, the eukaryotic has well enveloped nucleus and prokaryotic has nucleus without membrane which is also known as nucleoid
Eukaryotes have organized nucleus and prokaryotes don't have organized nucleus

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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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