Download Mendel`s Laws of Segregation

Biology
HS/Science
Unit: 08 Lesson: 02
Mendel's Law of Segregation
Mendel's hypothesis has four essential parts:
1. “Alternative versions of genes account for variations in inherited characters.”
This is the concept of alleles. Alleles are different versions of genes that control the same trait. For
example, each pea plant has two genes that control seed color. There are also two possible colors
(green or yellow), so there are two different genes for seed color.
2. “For each character trait (such as height, color, texture etc.), an organism inherits two genes, one
from each parent.”
This statement indirectly refers to the fact that when somatic cells (any cell of a living organism other
than the reproductive cells) are produced from two gametes, one allele comes from the mother and
one from the father. These alleles could be the same (true-breeding organisms/self-pollination) or
different (hybrids/cross-pollination).
3. “If the two alleles differ, then one, the dominant allele, is fully expressed in the organism's
appearance; the other, the recessive allele, has no noticeable effect on the organism's
appearance.”
©2012, TESCCC
11/05/12
page 1 of 2
Biology
HS/Science
Unit: 08 Lesson: 02
4. “The two genes for each character segregate during gamete production.”
This references meiosis, when the chromosome number changes from diploid to haploid (for
example, in humans from 46 to 23). The genes are sorted into separate gametes, resulting in
variation. “This sorting process depends on genetic “recombination”. During this time, genes mix and
match in a random and yet very specific way. Genes for each trait only trade with genes of the same
trait on the opposing strand of DNA so that all the traits are covered in the resulting offspring. For
example, color genes do not trade off with genes for texture.”
It was very important that Mendel observed inheritable traits that do not have intermediate forms
because that was the leading biological theory during his time. This theory stated that inherited traits
blended from generation to generation. (There are exceptions to this general rule. Some genes are
now known to have incomplete dominance. In this situation, the dominant gene has incomplete
expression in the resulting phenotype creating a “mixed” phenotype.)
Law of Independent Assortment
According to the principle* of independent assortment, different pairs of alleles are passed to
offspring independently of each other. The result is that new combinations of genes present in neither
parent are possible. For example, a pea plant's inheritance of the ability to grow tall instead of short
does not make it more likely that it will also inherit the ability to produce purple flowers instead of
white flowers. Likewise, the principle of independent assortment explains why the human inheritance
of a particular eye color does not increase or decrease the likelihood of having six fingers on each
hand. Today, we know this is due to the fact that the genes for independently assorted traits are
located on different chromosomes.
*Some biologists call Mendel’s theories “laws” and some call them “principles”.
Information adapted from: http://naturalselection.0catch.com/Files/gregormendel.html
©2012, TESCCC
11/05/12
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