Chapter 16 cont.
❖ Inheritance pattern of single traits
➢ In a typical breeding experiment, Mendel would cross-fertilize contrasting, true-
breeding pea varieties
■ True-breeding plants breed true for certain trait; offspring have same traits
as parents/previous generations
➢ Mendel would then allow the F1 hybrids to self-pollinate to produce an F2
generation
■ Parental (P) generation > first filial (F1) > Second Filial (F2)
➢ Mendel’s quantitative analysis of F2 plants resulted in two fundamental principles
of heredity: the law of segregation and the law of independent assortment
➢ In Mendel’s first experiment, he cross-pollinated two contrasting, true-breeding
pea varieties
■ Monohybrid cross- cross that follows only two variations of a single
character
■ White flowered plants x purple-flowered plants (or tall x dwarf)
■ All the F1 plants produced had purple flowers
■ Upon self pollination of F1 plants resulted in the F2 generation of plants
having either purple and white flowers in an approximately 3:1 ratio
respectively
● Purple is dominant to white
● White is recessive to purple
➢ From these experiments, Mendel made the following conclusions
■ Plants he crossed did not produce any offspring of intermediate
appearance (ie. blending does not occur)
■ For each pair of alternative forms of a trait, one alternative was not
expressed in the F1 hybrids
■ The pairs of alternative traits were segregated among the offspring, some
exhibiting one trait and some the other
■ These alternative traits were expressed in the F2 generation in the ratio of
¾ dominant to ¼ recessive (3:1 ratio)
➢ Based on his experiments, Mendel developed a model to explain the results
■ Traits may exist in two forms: dominant or recessive
■ An individual carries two genes for a given character, and genes have
variant forms (which are now called alleles)
● Not all copies of a gene are identical: alleles can be homozygous
or heterozygous
■ Two alleles of a gene separate during gamete formation so that each
sperm and egg receives only one allele
❖ Mendel’s law of segregation
➢ The two alleles of a gene separate (segregate) from each other during the
, process that gives rise to gametes, so every gamete only receives one allele
■ Rejoined at random, one from each parent, at fertilization
➢ Although Mendel did not know this at the time, we now know that the law of
segregation is a result of what happens during meiosis
■ Keep in mind that Mendel knew nothing of genes or chromosomes
❖ Punnett square
➢ The results from these experiments suggested to Mendel that fertilization is a
chance event with a number of possible outcomes
■ Based on this, he could predict what the chance is that an event will
happen
➢ A punnett square is a way to predict the outcome of a simple genetic cross
between individuals of known genotype
❖ Inheritance pattern of two characters
➢ Mendel conducted additional experiments in which he followed the inheritance of
two different characters
■ Dihybrid cross- a cross that follows two different characters
➢ In one dihybrid cross experiment, Mendel studied the inheritance of seed color
and shape
■ Yellow color (Y) is dominant to green color (y)
■ Round (R ) is dominant to wrinkled (r )
➢ Mendel crossed true breeding plants that had yellow and round seeds (YYRR)
with the true breeding seeds that had green and wrinkled seeds (yyrr)
❖ Dihybrid crosses
➢ Because the Y and R alleles are dominant, the F1 seeds were all round and
yellow
➢ The dihybrid crosses resulted in four different phenotypes appearing in the F2
generation in a ratio of about 9:3:3:1
➢ The parental traits appeared in new combinations (green round and yellow
wrinkled) not seen before
❖ Law of independent assortment
➢ These results suggested that the two pairs of alleles (Yy and Rr) segregated
independently of each other and not linked together in a package
■ The presence of one specific allele for one trait has no impact on the
presence of a specific allele for the second trait
■ Each character appeared to be inherited independently
➢ Mendel’s law of independent assortment- the alleles of different genes assort
independently of each other during the process that gives rise to gametes
■ This holds true for genes located on different chromosomes
❖ The chromosome theory of inheritance
➢ Chromosomes contain the genetic material (DNA)
■ Genes are found in the chromosomes
➢ Chromosomes are replicated and passed from parent to offspring
■ They are also passed from cell to cell during the development of a
multicellular organism
, ➢ The nucleus of a diploid cell contains two sets of chromosomes, found in
homologous pairs
■ Maternal and paternal sets of homologous chromosomes are functionally
equivalent; each set carries a full complement of genes
➢ At meiosis, one member of each chromosome pair segregates into each
daughter nucleus
■ During the formation of haploid cells, the members of different
chromosome pairs segregate independently of each other
➢ Gametes are haploid cells that combine to form a diploid cell during fertilization,
with each gamete transmitting one set of chromosomes to the offspring
❖ Sex chromosomes and sex determination
➢ In many organisms, sex-determining genes are located on specific chromosomes
called sex chromosomes
➢ All the other chromosomes are known as autosomal chromosomes (they are the
same in both sexes)
➢ In humans, genes that determine males are located on the Y chromosome
➢ Human males have XY and females have XX
❖ Sex chromosomes
➢ As a result of meiosis, all eggs will contain one X chromosome while half of the
sperm will contain one X chromosome or one Y chromosome
➢ The Y chromosome, being much smaller in size, has over 300 genes
■ One of these genes is the “master” gene for male sex determination (SRY
gene: sex determining region of Y)
■ If the gene is present, the testis will form
■ If it is absent, the ovaries will form
➢ The X chromosome, being much larger in size, has over 2,000 genes, most
involved in non-sexual traits
❖ Variations in inheritance patterns
➢ A number of assumptions are built into Mendel’s principles that are
oversimplifications
■ Simple Mendelian inheritance
➢ Heritable variations are often more complex than predicted by simple Mendelian
genetics
■ Pleiotropy
■ Incomplete dominance
■ Multiple alleles and codominance
■ The environment
❖ Pleiotropy
➢ Term used to describe the multiple effects that a gene may have on the
phenotype
■ An example is Phenylketonuria (PKU)
➢ PKU- one of the most common inherited disorders, occurring in approx. 1 in
10,000 babies born in the US
➢ It occurs in babies who inherit two mutant genes (homozygous recessive) for the
❖ Inheritance pattern of single traits
➢ In a typical breeding experiment, Mendel would cross-fertilize contrasting, true-
breeding pea varieties
■ True-breeding plants breed true for certain trait; offspring have same traits
as parents/previous generations
➢ Mendel would then allow the F1 hybrids to self-pollinate to produce an F2
generation
■ Parental (P) generation > first filial (F1) > Second Filial (F2)
➢ Mendel’s quantitative analysis of F2 plants resulted in two fundamental principles
of heredity: the law of segregation and the law of independent assortment
➢ In Mendel’s first experiment, he cross-pollinated two contrasting, true-breeding
pea varieties
■ Monohybrid cross- cross that follows only two variations of a single
character
■ White flowered plants x purple-flowered plants (or tall x dwarf)
■ All the F1 plants produced had purple flowers
■ Upon self pollination of F1 plants resulted in the F2 generation of plants
having either purple and white flowers in an approximately 3:1 ratio
respectively
● Purple is dominant to white
● White is recessive to purple
➢ From these experiments, Mendel made the following conclusions
■ Plants he crossed did not produce any offspring of intermediate
appearance (ie. blending does not occur)
■ For each pair of alternative forms of a trait, one alternative was not
expressed in the F1 hybrids
■ The pairs of alternative traits were segregated among the offspring, some
exhibiting one trait and some the other
■ These alternative traits were expressed in the F2 generation in the ratio of
¾ dominant to ¼ recessive (3:1 ratio)
➢ Based on his experiments, Mendel developed a model to explain the results
■ Traits may exist in two forms: dominant or recessive
■ An individual carries two genes for a given character, and genes have
variant forms (which are now called alleles)
● Not all copies of a gene are identical: alleles can be homozygous
or heterozygous
■ Two alleles of a gene separate during gamete formation so that each
sperm and egg receives only one allele
❖ Mendel’s law of segregation
➢ The two alleles of a gene separate (segregate) from each other during the
, process that gives rise to gametes, so every gamete only receives one allele
■ Rejoined at random, one from each parent, at fertilization
➢ Although Mendel did not know this at the time, we now know that the law of
segregation is a result of what happens during meiosis
■ Keep in mind that Mendel knew nothing of genes or chromosomes
❖ Punnett square
➢ The results from these experiments suggested to Mendel that fertilization is a
chance event with a number of possible outcomes
■ Based on this, he could predict what the chance is that an event will
happen
➢ A punnett square is a way to predict the outcome of a simple genetic cross
between individuals of known genotype
❖ Inheritance pattern of two characters
➢ Mendel conducted additional experiments in which he followed the inheritance of
two different characters
■ Dihybrid cross- a cross that follows two different characters
➢ In one dihybrid cross experiment, Mendel studied the inheritance of seed color
and shape
■ Yellow color (Y) is dominant to green color (y)
■ Round (R ) is dominant to wrinkled (r )
➢ Mendel crossed true breeding plants that had yellow and round seeds (YYRR)
with the true breeding seeds that had green and wrinkled seeds (yyrr)
❖ Dihybrid crosses
➢ Because the Y and R alleles are dominant, the F1 seeds were all round and
yellow
➢ The dihybrid crosses resulted in four different phenotypes appearing in the F2
generation in a ratio of about 9:3:3:1
➢ The parental traits appeared in new combinations (green round and yellow
wrinkled) not seen before
❖ Law of independent assortment
➢ These results suggested that the two pairs of alleles (Yy and Rr) segregated
independently of each other and not linked together in a package
■ The presence of one specific allele for one trait has no impact on the
presence of a specific allele for the second trait
■ Each character appeared to be inherited independently
➢ Mendel’s law of independent assortment- the alleles of different genes assort
independently of each other during the process that gives rise to gametes
■ This holds true for genes located on different chromosomes
❖ The chromosome theory of inheritance
➢ Chromosomes contain the genetic material (DNA)
■ Genes are found in the chromosomes
➢ Chromosomes are replicated and passed from parent to offspring
■ They are also passed from cell to cell during the development of a
multicellular organism
, ➢ The nucleus of a diploid cell contains two sets of chromosomes, found in
homologous pairs
■ Maternal and paternal sets of homologous chromosomes are functionally
equivalent; each set carries a full complement of genes
➢ At meiosis, one member of each chromosome pair segregates into each
daughter nucleus
■ During the formation of haploid cells, the members of different
chromosome pairs segregate independently of each other
➢ Gametes are haploid cells that combine to form a diploid cell during fertilization,
with each gamete transmitting one set of chromosomes to the offspring
❖ Sex chromosomes and sex determination
➢ In many organisms, sex-determining genes are located on specific chromosomes
called sex chromosomes
➢ All the other chromosomes are known as autosomal chromosomes (they are the
same in both sexes)
➢ In humans, genes that determine males are located on the Y chromosome
➢ Human males have XY and females have XX
❖ Sex chromosomes
➢ As a result of meiosis, all eggs will contain one X chromosome while half of the
sperm will contain one X chromosome or one Y chromosome
➢ The Y chromosome, being much smaller in size, has over 300 genes
■ One of these genes is the “master” gene for male sex determination (SRY
gene: sex determining region of Y)
■ If the gene is present, the testis will form
■ If it is absent, the ovaries will form
➢ The X chromosome, being much larger in size, has over 2,000 genes, most
involved in non-sexual traits
❖ Variations in inheritance patterns
➢ A number of assumptions are built into Mendel’s principles that are
oversimplifications
■ Simple Mendelian inheritance
➢ Heritable variations are often more complex than predicted by simple Mendelian
genetics
■ Pleiotropy
■ Incomplete dominance
■ Multiple alleles and codominance
■ The environment
❖ Pleiotropy
➢ Term used to describe the multiple effects that a gene may have on the
phenotype
■ An example is Phenylketonuria (PKU)
➢ PKU- one of the most common inherited disorders, occurring in approx. 1 in
10,000 babies born in the US
➢ It occurs in babies who inherit two mutant genes (homozygous recessive) for the
