Unit 5 Objectives
Patterns of Inheritance
CHAPTER
9
Chapter Objectives
Mendel’s Laws
9.1
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Describe pangenesis theory and the blending hypothesis. Explain why both ideas are now
rejected.
Pangenesis theory stated that particles called pangenes travel from each part of an organism’s body to the
eggs or sperm and then are passed to the next generation; moreover, changes that occur in the body during
an organism’s life are passed on in this way.
o Suggested by a physician from ancient Greece named Hippocrates.
The Greek philosopher Aristotle rejected this idea as simplistic, saying that what is inherited is the potential
to produce body features rather than particles of the features themselves.
Pangenesis is incorrect because the reproductive cells are not composed of particales from somatic (body)
cells, and changes in somatic cells do not influence eggs and sperm.
The blending hypothesis is the idea that the hereditary materials contributed by the male and female parents
mix in forming the offspring similar to the way that blue and yellow paints blend to make green.
Rejected because it does not explain how traits that disappear in one generation can reappear in later ones.
9.2 Explain why Mendel’s decision to work with peas was a good choice. Define and distinguish
between true-breeding organisms, hybrids, the P generation, the F1 generation, and the F2
generation.
His decision to work with peas was a good choice because they had short generation times, produced large
numbers of offspring from each mating, and came in many readily distinguishable varieties. He could also
could strictly control matings.
Hybrids are the offspring of two different varieties, and the cross-fertilization itself is referred to as a
hybridization, or simply a genetic cross.
P generation are the true breeding parental plants (P for parental), and their hybrid offspring are called the F1
generation (F for filial, form the Latin word for “son”).
F2 generation are when the F1 plants self-fertilize or fertilize each other.
True-breeding varieties is when you have varieties for which self-fertilization roduced offspring all
identical to the parent.
9.3 Define and distinguish between the following pairs of terms: homozygous and heterozygous;
dominant allele and recessive allele; genotype and phenotype. Also, define a monohybrid cross
and a Punnett square.
Alleles are the alternative versions of a gene
Homozygous and heterozygous:
o An organism that has two identical alleles for a gene is said to be homozygous for that tgene (and is
a “homozygote” for that trait).
o An organism that has two different alleles fro a gene is said to be heterozygous for that gene (and is
a “heterozygote”).
Dominant allele and recessive allele:
o If the two alleles of an inherited pair differ, then one determines the organism’s appearance and is
called the dominat allele
o The other has no noticeable effect on the organism’s appearance and iscalled the recessive allele.
Genotype and phenotype:
o An organism’s physical traits is called its phenotype
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o An organism’s genetic makeup is its genotype.
Monohybrid cross:
o When the parent plants differ in only one character.
Punnett square:
o Shows the possible combinations of alleles that could occur when a group of gametes are combined.
9.3 Explain how Mendel’s law of segregation describes the inheritance of a single characteristic.
There are alternative versions of genes that account for variations in inherited characters
For each character, an organism inherits two alleles, one from each parent.
If the two alleles of an inherited pair difer, then one determines the organism’s appearance and is called the
dominant allele; the other has no noticeable effect on the organism’s appearance and is called the recessive
allele.
A sperm or egg carries only one allele for each inherited character because allele pairs separate (segregate)
from each other during the production of gametes.
9.4 Describe the genetic relationships between homologous chromosomes.
Homologous chromosomes have genes for the same characters located at the same positions along their
lengths.
However two homologous chromosmes may bear either the same alleles or different ones.
9.5 Explain how Mendel’s law of independent assortment applies to a dihybrid cross. Illustrate this
law with examples from Labrador retrievers and Mendel’s work with peas.
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9.6
9.7
Explain how a testcross is performed to determine the genotype of an organism.
Explain how and when the rule of multiplication and the rule of addition can be used to
determine the probability of an event. Explain why Mendel was wise to use large sample sizes in
his studies.
9.8 Explain how family pedigrees can help determine the inheritance of many
human traits.
9.9 Explain how recessive and dominant disorders are inherited. Provide examples of each.
9.10 Compare the health risks, advantages, and disadvantages of the following forms of fetal testing:
amniocentesis, chorionic villus sampling, and ultrasound imaging. Describe the ethical dilemmas
created by advances in biotechnology
discussed in this chapter.
Variations on Mendel’s Laws
9.11–9.15 Describe the inheritance patterns of incomplete dominance, multiple alleles, codominance,
pleiotropy, and polygenic inheritance. Provide an example of each.
9.13 Explain how the sickle-cell allele can be adaptive.
9.14–9.15 Explain why human skin coloration is not sufficiently explained by polygenic inheritance.
The Chromosomal Basis of Inheritance
9.16 Define the chromosome theory of inheritance. Explain the chromosomal basis of the laws of
segregation and independent assortment.
9.17 Explain how linked genes are inherited differently from nonlinked genes.
9.18 Describe T. H. Morgan’s studies of crossing over in fruit flies. Explain how crossing over
produces new combinations of alleles.
9.19 Explain how Sturtevant created linkage maps.
Sex Chromosomes and Sex-Linked Genes
9.20 Explain how sex is genetically determined in humans and the significance of the SRY gene.
Compare the sex determination system in humans to those in fruit flies, grasshoppers, birds, and
bees.
9.21–9.22 Describe patterns of sex-linked inheritance, noting examples in fruit flies and humans.
9.22 Explain why sex-linked disorders are expressed more frequently in men than in women.