Biology Summary
Chapter 4
4.1
Traits are distinguishing characteristics that make living organisms a unique
individual
Inherited is having certain physical traits that are transmitted from one
generation to the next
Genetics deals with the principles of variation and inheritance in animals and
plants
- study of genetics gives us greater understanding of how we can determine
the likelihood of inheriting certain traits
4.2
-
Gregor Mendel was the first to explain the inheritance of characteristics
He succeeded in discovering the basis of inheritance when others before
him had failed because:
1. chose the appropriate organism to study
- chose pea plant to study
- it’s commercially available
- easy to grow
- can self-pollinate so Mendel could control which plants reproduced
- different varieties of the pea had different traits that could be
observed easily from one generation to the next
2. designed and performed the experiment correctly
- obtained purebred plants for the trait he wanted to study
- Mendel produced purebred varieties of pea plants through selective
breeding to produce plants that were true breeding
3. analyzed the data properly
True Breeding describes organisms that are homozygous for a particular trait or
sets of traits and produce like offspring
Purebred organism s descended from ancestors of a distinct type or breed
P generation = parent generation
Hybrids are results of a cross between different purebred plants and are
heterozygous
Monohybrid is a cross of two heterozygous individuals that differ in one trait
Dominant trait is a characteristic that is always expressed
Recessive trait is a characteristic that is latent (present but inactive) and
therefore not usually expressed
Principle of dominance states that when individuals with contrasting traits are
crossed, the offspring will express only the dominant trait
Mendelian ratio is the ratio of dominant phenotype (homozygous dominant
genotype and heterozygous genotypes) to recessive phenotype (homozygous
recessive genotype)
Law of segregation states that inherited traits are determined by pairs of alleles
from each parent. The alleles separate during gamete formation giving each
offspring only one allele from each parent
Genes are parts of the chromosome that governs the expression of a particular
trait
Alleles are alternate form of a gene located at a particular place on a
chromosome
Homozygous is having two alleles for a trait that are the same, as is the result of
pure breeding ie tt
Heterozygous is having two alleles for a trait that are different ie Tt
Probability is the chance or likelihood of a particular outcome usually expressed
as a ratio
- the probability that two or more independent events will occur one after
another is the product of their individual probabilities of occurring alone
- ie the probability of flipping a coin twice and getting tail both times is ¼
- the chance of getting one tail is ½, the chance of getting the second tail is
also ½
- the chance of getting tail one after another is ½ x ½ = ¼
- the law of probability forms the basis of solving genetic problems
Punnett square is a simple grid used to illustrate all possible combinations of
gametes from a given set of parents
Genotype is the genetic make-up of an organism
Phenotype is the appearance of the trait in an organism
Mendelian ratio for genotype is the ratio of homozygous dominant genotype to
heterozygous genotypes to homozygous recessive genotype
Ie 1:2:1, there is 1 homozygous dominant genotype, 2 heterozygous genotypes
and 1 homozygous recessive genotype
Pedigree is a diagram that illustrates the genetic relationships among a group of
individuals
- careful analysis of different pedigrees has shown that certain human traits
are inherited as simple dominant traits
Simple dominant traits is trait that has only two possible alleles (dominant or
recessive) and is not influenced by the inheritance of other simple dominant traits
Carrier is an organism that is heterozygous for the given trait but does not show
the recessive trait
Test cross is used to determine the genotype of an individual
- involves crossing an individual of unknown genotype with a homozygous
recessive individual
- the offspring will exhibit certain phenotypes that will allow you to determine
if the unknown is homozygous dominant or heterozygous
- test crosses are valuable for determining whether a particular trait is
passed by one, two, or more pairs of alleles
- it’s important to know how many pairs of alleles govern a particular trait
because multiple allele inheritance is often influenced by environmental
factors
4.3
Dihybrid cross is a cross of two heterozygous individuals that differ in two traits
ie AaBb x AaBb
Law of Independent Assortment is the second law of inheritance states that
the inheritance of alleles for one trait does not affect the inheritance of alleles for
another trait
-
-
a test cross may also be used to determine the genotype of an individual
for two traits
a two trait test cross involves crossing an individual that shows the
dominant phenotype for two traits with an individual that is homozygous
recessive for the same two traits
the individual showing the dominant phenotype for both traits may be
either heterozygous or homozygous dominant
a homozygous recessive individual is used in the cross because it
provides the best chance of producing an offspring that is homozygous
recessive for both traits
4.4
-
not all traits are purely dominant or purely recessive
in some cases neither of the alleles controlling the trait are dominant
Incomplete dominance is an inheritance pattern where heterozygote exhibits an
intermediate phenotype between the two homologous phenotypes; neither allele
for the trait is completely dominant
- examples can be found in many species of plants ie snapdragon
-
in some cases both alleles for a trait may be dominant
Co-dominant is the case when both alleles for a trait are dominant
Multiple alleles is a pattern of inheritance when a gene may have more than two
alleles for any given trait