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>Principles of inheritance >
>Mendelian inheritance refers to the pattern of inheritance of genetic
traits that follow specific rules first described by Gregor Mendel, an
Austrian monk and scientist, in the 19th century. These rules describe how
certain traits are passed down from one generation to the next through the
inheritance of genes. >
>The principles of Mendelian inheritance include: >
>Law of Segregation: This states that an individual has two copies of each
gene, and these copies segregate during the formation of gametes (eggs and
sperm) so that each gamete receives only one copy. >
>Law of Independent Assortment: This states that the inheritance of one gene
does not influence the inheritance of another gene, and genes assort
independently during gamete formation. >
>Dominance and Recessiveness: This refers to the fact that some alleles
(versions of a gene) are dominant over others, meaning that they are
expressed in the phenotype (physical appearance), while recessive alleles
are only expressed in the phenotype when two copies are present. >
>These principles of Mendelian inheritance provide a foundation for
understanding how genetic traits are passed down from one generation to the
next and can be used to predict the probability of certain traits appearing
in offspring. >
>Law of Dominance first law >
>The Law of Dominance is one of the three fundamental principles of
Mendelian inheritance. It states that when two different alleles (variations
of a gene) are present in an individual, one of them (the dominant allele)
will be expressed in the phenotype (physical appearance), while the other
(the recessive allele) will be masked or not expressed. >
>For example, let's consider the gene for flower color in pea plants. The
gene has two alleles: one for purple flowers (P) and one for white flowers
(p). When a pea plant has two copies of the dominant allele (PP), it will
have purple flowers. If it has one copy of the dominant allele and one copy
of the recessive allele (Pp), it will still have purple flowers because the
dominant allele masks the recessive allele. Only when a pea plant has two
copies of the recessive allele (pp) will it have white flowers, because
there is no dominant allele present to mask the recessive allele. >
>This pattern of inheritance is seen in many different traits in organisms,
including human traits like eye color and blood type. The Law of Dominance
helps explain why some traits are more common than others and how genetic
variation is maintained in populations. >
or
>The law of dominance can also be illustrated with the example of plant
height in pea plants. >
>In pea plants, plant height is determined by a single gene with two
possible alleles: the dominant allele (T) for tall plants and the recessive
allele (t) for dwarf plants. When a pea plant inherits two dominant alleles
(TT) or one dominant and one recessive allele (Tt), it will be tall because
the dominant allele is expressed. Only when a pea plant inherits two
recessive alleles (tt) will it be dwarf, because there is no dominant allele
to mask the expression of the recessive allele. >
>For example, if a pea plant with a heterozygous genotype (Tt) for plant
height is crossed with another pea plant with the same genotype, the Punnett
square for the cross predicts that 25% of the offspring will have the
homozygous dominant genotype (TT) and be tall, 50% of the offspring will
have the heterozygous genotype (Tt) and also be tall, and 25% of the
offspring will have the homozygous recessive genotype (tt) and be
dwarf. >
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>As you can see, all of the F1 offspring are tall because they inherit one
dominant allele (T) from the tall parent. However, they are all heterozygous
(Tt) because they also inherit one recessive allele (t) from the dwarf
parent.>
>As predicted by the law of dominance, we see a 3:1 ratio of tall to dwarf
offspring, with the tall offspring being either heterozygous (Tt) or
homozygous dominant (TT), and the dwarf offspring being homozygous
recessive (tt). This is because the dominant allele (T) is always
expressed in the heterozygous individuals (Tt).
>Law of Segregation >
>The Law of Segregation is one of the three fundamental principles of
Mendelian inheritance. It states that when gametes (eggs and sperm) are
formed during sexual reproduction, the two alleles (versions of a gene)
that an individual possesses for a particular trait separate from each
other and only one allele is passed down to each gamete. >
>For example, let's consider the gene for flower color in pea plants.
The gene has two alleles: one for yellow flowers (Y) and one for green
flowers (y). When a pea plant with genotype Yy produces gametes, the two
alleles separate from each other during the formation of eggs and sperm,
so that each gamete receives only one allele. Thus, half of the gametes
will contain the Y allele and the other half will contain the y allele.
When the gametes combine during fertilization, the resulting offspring
will inherit one allele from each parent, and the genotype of the
offspring will be determined by which alleles they receive. >
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>The Law of Segregation helps explain how genetic variation is passed
down from generation to generation and how different combinations of
alleles can lead to different phenotypes (physical appearances) in
offspring. It is an important principle of genetics that has been
verified by countless experiments and observations. >
>Law of independent assortment >
>The law of independent assortment is a principle of genetics that
states that the inheritance of one trait does not affect the inheritance
of another trait. This means that when two or more traits are inherited,
they are inherited independently of each other. >
>This law was first proposed by Gregor Mendel, the father of modern
genetics, in the mid-1800s. Mendel conducted experiments with pea plants
and found that the inheritance of one trait, such as seed color, was
independent of the inheritance of another trait, such as seed
shape. >
>Let's look at a concrete example of the law of independent assortment.
Imagine that we cross two pure-breeding pea plants: one with yellow,
round seeds (YYRR) and one with green, wrinkled seeds (yyrr). Because
each parent is homozygous, the law of segregation tells us that the
gametes made by the wrinkled, green plant all are ry, and the gametes
made by the round, yellow plant are all RY. That gives us F1 offspring
that are all RrYy. >
>The allele specifying yellow seed color is dominant to the allele
specifying green seed color, and the allele specifying round shape is
dominant to the allele specifying wrinkled shape, as shown by the
capital and lower-case letters. This means that the F1 plants are all
yellow and round. Because they are heterozygous for two genes, the F1
plants are called dihybrids (di- = two, -hybrid = heterozygous). >
>A cross between two dihybrids (or, equivalently, self-fertilization of
a dihybrid) is known as a dihybrid cross. When Mendel did this cross and
looked at the offspring, he found that there were four different
categories of pea seeds: yellow and round, yellow and wrinkled, green
and round, and green and wrinkled. These phenotypic categories
(categories defined by observable traits) appeared in a ratio of
approximately 9:3:3:1. >
>This ratio was the key clue that led Mendel to the law of independent
assortment. That's because a 9:3:3:1 ratio is exactly what we'd expect
to see if the F1 plant made four types of gametes (sperm and eggs) with
equal frequency: YR, Yr, yR, and yr. In other words, this is the result
we'd predict if each gamete randomly got a Y or y allele, and, in a
separate process, also randomly got an R or r allele (making four
equally probable combinations). >
>We can confirm the link between the four types of gametes and the
9:3:3:1 ratio using the Punnett square above. To make the square, we
first put the four equally probable gamete types along each axis. Then,
we join gametes on the axes in the boxes of the chart, representing
fertilization events. The 16 equal-probability fertilization events that
can occur among the gametes are shown in the 16 boxes. The offspring
genotypes in the boxes correspond to a 9:3:3:1 ratio of phenotypes, just
as Mendel observed. >
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>The law of independent assortment applies to genes located on different
chromosomes or to genes located far apart on the same chromosome.
However, genes located close together on the same chromosome are more
likely to be inherited together, a phenomenon known as genetic
linkage. >
>The law of independent assortment has important implications for
understanding the inheritance of genetic traits and for predicting the
outcomes of genetic crosses. It allows geneticists to make predictions
about the likelihood of particular traits appearing in offspring, based
on the known genetic makeup of the parents. >
> >Mendelian inheritance patterns | > >Mendel's laws of genetics | > >Dominant and recessive traits | > >Punnett squares and genetic crosses | > >Inheritance of sex-linked traits | > >Genetic variation and probability | > >Genotype and phenotype ratios
> >Mendelian genetics and human disease | > >Genetic counseling and inheritance patterns | > >Genetic testing and Mendelian disorders > >