Transcript General Genetics

General Genetics
Chapter 14
Mendel and the Gene Idea
Objectives
• Understand Mendel's three Laws governing genetics
• Understand the meaning of the relevant vocabulary
discussed in class
• Be able to predict the results of a mono & dihybrid cross
using a Punnett square
• Understand how recombination of genes affect genetic
variability
• Be familiar with special situations regarding
genotype/phenotype predictions
Terms from Chapter 13
• Diploid: having 2 of each chromosome type
• Haploid: having a single representative of
each chromosome type
• Homologous chromosomes (homologs):
chromosomes from different parents that are
of the same type (contain similar
information)
• Sister Chromatids: 2 “identical” strands of
DNA connected by a centromere that
contains a kinetochore. Makes up each
member of a homologous pair
Descriptors
• The physical
appearance of an
organism reflects its
genetic makeup
• Each gene codes for a
different polypeptide
• Polypeptide
combinations may
alter the appearance of
an organism
• Gene (Character): is a feature
that is heritable
• Locus: specific area on
chromosome where the gene
is found
• Allele (Trait): is a variation of
a character
• Genotype: the genetic
makeup of an organism
(combination of genes in its
nucleus)
• Phenotype: the physical
appearance of an organism
Mendelian Genetics
• Gregor Mendel described
three Laws of Genetics
• Law of Segregation: each
parent has two copies of a
gene but only one is
passed to the offspring via
the gametes (separation of
homologous pairs)
• Three possible genotypes
for each gene in the
diploid cell
• Homozygous dominant:
both alleles of a gene are
of the “Dominant” variety
• Homozygous recessive:
both alleles of a gene are
of the “Recessive” variety
• Heterozygous: the diploid
cell has one dominant and
one recessive allele for
each gene
• Law of Independent
Assortment: genes
residing on different
chromosomes separate
without regard for one
another
– describes the broad range of
variation seen in organisms
• Law of Dominance: some
alleles for a gene are fully
expressed if present
(dominant) in the
phenotype while others
may have their effect
masked (recessive)
– hierarchy of alleles
Genetics is
Probability
• In a diploid organism
(2n), each allele has a
50:50 chance of being
found in a particular
gamete (1/2).
• To calculate the
likelihood of two
alleles recombining
during fertilization, we
must multiply our
probability for each
allele together
(1/2 x 1/2 = 1/4)
Punnett Square
• Device used to predict potential genotypes
of offspring
• Along each axis are placed the gamete
possibilities for each parent
• Internal boxes represent union of genotypes
for offspring produced by the union of the
corresponding axial gametes
• Phenotypes can be determined for each
potential zygote
Special Situations
• Incomplete dominance:
the phenotype of a
heterozygous genotype is
intermediate in
appearance
• Codominance: each allele
in the genotype for a
particular gene will be
expressed in the
phenotype
• Pleiotropy: the ability
of a gene to affect an
organism in many
ways
• Epistasis: gene at one
locus influences the
expression of a gene at
another locus
(different gene)
• Polygenic
Inheritance: additive
effect of 2 or more
genes on a phenotypic
character
Incomplete Dominance
(heterozygous genotype is intermediate in appearance)
Codominance
(each genotypic allele will be expressed)
Pleiotropy
(single gene affects multiple phenotypes)
• Sickle Cell Disease
– Production of
abnormal hemoglobin
• Prevalent amongst
African Americans,
but rare in people of
other races
Epistasis
(one gene influences the expression of another gene)
Polygenic Inheritance
(additive effect of 2 or more genes on a phenotypic character)