1. Mendelian Genetics
CH 11

2. Question Set 1: Terms of Inheritance
What is a homologous chromosome?
What is a loci?
What is an allele?
What is a genotype?
What is a gene?
What is a trait?
How many alleles does a genotype for one trait include?
How many alleles does a genotype for two traits include?
How many alleles do you get from each parent for a trait?

3. Inheritance in Modern Terms
Today, we know that individuals of a species share certain traits because their chromosomes carry the same genes
The DNA sequence of each gene occurs at a specific location (locus) on a particular chromosome
locus
Location of a gene on a chromosome

4. Loci of Some Human Genes

5. Modern Terms (cont.)
Diploid cells have pairs of genes, on pairs of homologous chromosomes
The two genes of a pair may be identical (homozygous), or they may be slightly different alleles (heterozygous)
homozygous
Having identical alleles of a gene
heterozygous
Having two different alleles of a gene

6. Genes on Chromosomes
Any pair of genes on homologous chromosomes may vary as alleles
Different alleles may result in different versions of a trait

7. Modern Terms (cont.)
The particular set of alleles that an individual carries is their genotype, and their observable traits, such as flower color, make up their phenotype
genotype
The particular set of alleles carried by an individual
phenotype
An individual’s observable traits

8. Modern Terms (cont.)
Offspring resulting from a cross, or mating, between homozygous individuals with different traits are hybrids
Often, expression of one allele influences expression of the other, and the outcome is visible in the hybrid phenotype
hybrid
Offspring of a cross between two individuals that breed true for different forms of a trait; a heterozygous individual

9. Modern Terms (cont.)
An allele is dominant when its effect masks that of a recessive allele paired with it
dominant
An allele that masks the effect of a recessive allele paired with it (indicated by italic capital letters: A)
recessive
An allele whose effect is masked by a dominant allele on the homologous chromosome (italic lowercase: a)

10. Question Set 1: Terms of Inheritance
What is a homologous chromosome?
What is a loci?
What is an allele?
What is a genotype?
What is a gene?
What is a trait?
How many alleles does a genotype for one trait include?
How many alleles does a genotype for two traits include?
How many alleles do you get from each parent for a trait?

11. Question Set 2: Mendel’s Law of Segregation
Why did Mendel work on Pea plants?
What is Mendel’s Law of Segregation?
During what process does segregation occur?
What 2 processes are diagramed when making a Punnett square?
What evidence supports the law of segregation?

12. Key Concepts Where Modern Genetics Started
Gregor Mendel gathered evidence of the genetic basis of inheritance
His meticulous work gave him clues that heritable traits are specified in units
The units, which are distributed into gametes in predictable patterns, were later identified as genes

13. 13.2 Mendel, Pea Plants, and Inheritance Patterns
Gregor Mendel, an Austrian monk, carefully documented how certain traits in pea plants are inherited

14. Mendel’s Experimental Approach
Mendel started with garden pea plants that “bred true” for a particular trait, meaning that trait stayed the same in all descendants, generation after generation
When Mendel cross-fertilized pea plants with different traits, the traits of the offspring appeared in predictable patterns
Mendel concluded that hereditary information is passed from one generation to the next in discrete units

15. Breeding Garden Peas B A carpel anther
A Garden pea flower, cut in half. Male gametes form
in pollen grains produced by the anthers, and female gametes form in carpels. Experimenters can control
the transfer of hereditary material from one flower to another by snipping off a flower’s anthers (to prevent the flower from self-fertilizing), and then brushing pollen from another flower onto its carpel. C
Figure Breeding garden pea plants (Pisum sativum), which can self-fertilize or cross-fertilize.
B In this example, pollen from a plant with purple flowers is brushed onto the carpel of a white-flowered plant.
C Later, seeds develop inside pods of the cross-fertilized plant. An embryo in each seed develops
into a mature pea plant.
D Every plant that arises from this cross has
purple flowers. Predictable patterns such as
this offer evidence of how inheritance works. D
Fig. 13.2, p. 190

16. 13.3 Mendel’s Law of Segregation
When Mendel crossed plants that bred true for purple flowers with plants that bred true for white flowers, all of the offspring had purple flowers
A pea plant with two P alleles (PP) has purple flowers, and one with two p alleles (pp) has white flowers
The allele for purple (P) is dominant over the allele for white (p), so the heterozygote (Pp) also has purple flowers

17. Law of Segregation (cont.)
When homologous chromosomes separate during meiosis, the gene pairs on those chromosomes separate also
Each gamete that forms carries only one gene of a pair
Homozygous dominant plants (PP) make (P) gametes
Homozygous recessive plants (pp) make (p) gametes
Heterozygous plants (Pp) make equal numbers of (P) and (p) gametes

18. Law of Segregation (cont.)
When homozygous dominant and homozygous recessive plants are crossed (PP X pp), only one outcome is possible
A gamete with a P allele meets a gamete with a p allele, and all first generation (F1) offspring will be heterozygous
Genotype = Pp
Phenotype = purple

19. Gene Segregation DNA replication meiosis I gametes (p) meiosis II1 2
gametes (p)
meiosis II
gametes (P)
zygote (Pp) 3
Figure 13.5 Gene segregation. Homologous chromosomes separate during meiosis, so the pairs of genes they carry separate too. Each of the resulting gametes carries only one of the two members of each gene pair. For clarity, we show only one set of homologous chromosomes.
female gametes
male gametes 4
Stepped Art
Fig. 13.5, p. 192

20. Punnett Squares Punnett square
Diagram used to predict the genetic and phenotypic outcome of a cross

21. Testcross
Breeding experiments use testcrosses to determine whether the tested individual is heterozygous or homozygous
testcross
Method of determining genotype in which an individual of unknown genotype is crossed with one that is known to be homozygous recessive

22. Monohybrid Cross
Another breeding experiment, a monohybrid cross, checks the dominance relationship for a single trait
monohybrid cross
Breeding experiment in which individuals identically heterozygous for one gene are crossed
Frequency of traits among offspring offers information about the dominance relationship between the alleles

23. A Monohybrid Cross
In a monohybrid cross between two Pp plants (Pp X Pp), the two types of gametes can meet in four possible ways:
Sperm P meets egg P → zygote genotype PP
Sperm P meets egg p → zygote genotype Pp
Sperm p meets egg P → zygote genotype Pp
Sperm p meets egg p → zygote genotype pp
The probability that second-generation (F2) offspring will have purple flowers is 3 purple to 1 white, or a ratio of 3:1

24. parent plant homozygous for purple flowers
two types of gametes Pp
hybrid
parent plant
homozygous
for white
flowers
parent plant homozygous for purple flowers p pp P PP
A All of the F1 offspring of a cross between two plants that breed true for different forms of a trait are identically heterozygous. These offspring make two types of gametes: P and p
A Monohybrid Cross
B A cross between the F1 offspring is a monohybrid cross. The phenotype ratio in F2 offspring in this example is 3:1 (3 purple to 1 white). P p PP Pp Pp pp
Figure Example of a monohybrid cross.
Stepped Art
Fig. 13.6, p. 193

25. Traits of Pea Plants
Mendel found that all these traits of pea plants were inherited in the same pattern in F1 and F2 generations

26. Mendel’s Law of Segregation
The 3:1 phenotype ratios in F2 offspring of monohybrid crosses became the basis of Mendel’s law of segregation
law of segregation
The two members of each pair of genes on homologous chromosomes end up in different gametes during meiosis

27. Key Concepts Insights From Monohybrid Crosses
During meiosis, pairs of genes on homologous chromosomes separate and end up in different gametes
Inheritance patterns of alleles associated with different forms of a trait can be used as evidence of such gene segregation

28. Question Set 2: Mendel’s Law of Segregation
Why did Mendel work on Pea plants?
What is Mendel’s Law of Segregation?
During what process does segregation occur?
What 2 processes are diagramed when making a Punnett square?
What evidence supports the law of segregation?