13. Mendelian Genetics

Test Crosses

Test Crosses | Genetics

In flies, small wings are recessive to normal wings. If a cross between two flies produces 8 small-wing offspring and 28 normal-wing offspring, what are the most likely genotypes of the parents? (Use S to represent the normal-wing allele and s to represent the short-wing allele.)

You want to determine the pea color genotype of a pea plant with yellow peas. You conduct a test cross with your mystery pea plant. The test cross results in 50% of the offspring possessing yellow peas and 50% of the offspring possessing green peas. What is the genotype for pea color of the mystery parent?a) YY.b) Yy.c) yy.d) None of the above.

The size and shape of the vertebrate skull can reveal a great deal about an animal’s lifestyle and evolutionary relationships. Consider your own skull. If you put your finger in your ear and move your jaw up and down, you can feel the space near the hinge of your jaw. Nestled in this space are the tiny bones that make your hearing possible: the malleus, incus, and stapes. All mammals have these three ear bones, but reptiles such as this T. rex don’t. Where did ear bones come from?

Gene expression patterns can be used to test hypotheses based on morphology. For example, the regulatory gene Bapx1 is expressed in the hinge of the developing lower jaw in fishes and reptiles. Where would you predict Bapx1 expression to occur in mammals?

Animation: Testcross in "MendAliens"

Monohybrid Test Cross (Mendel's Experiments)

Steps to Performing a Test Cross

 gray-bodied, normal-winged; black-bodied, vestigial-winged 

 No. Gray-bodied, normal-winged is not a parental type. 

 gray-bodied, vestigial-winged; black-bodied, normal-winged 

 No. Black-bodied, normal-winged is not a recombinant type. 

 gray-bodied, normal-winged; black-bodied, normal-winged 

 No. Gray-bodied, normal-winged is not a parental type, and black-bodied, normal-winged is not a recombinant type. 

 black-bodied, normal-winged; black-bodied, normal-winged 

 No. Black-bodied, normal-winged is a parental type and not a recombinant type. 

 black-bodied, normal-winged; black-bodied, vestigial-winged 

There are two parental types and two recombinant types.

A gray-bodied, vestigial-winged fly is crossed with a black-bodied, normal-winged fly. The F1 progeny is testcrossed. Among the resulting offspring, __________ is a parental type, and __________ is a recombinant type.

A single gene test cross is conducted to determine the genotype of a pea plant that shows the dominant phenotype for height (T = tall, t = short). If all offspring of the cross show the dominant phenotype, then the genotype of the unknown parent is _______.a) TT.b) tT.&nbsp;c) Tt.&nbsp;d) tt.

The size and shape of the vertebrate skull can reveal a great deal about an animal’s lifestyle and evolutionary relationships. Consider your own skull. If you put your finger in your ear and move your jaw up and down, you can feel the space near the hinge of your jaw. Nestled in this space are the tiny bones that make your hearing possible: the malleus, incus, and stapes. All mammals have these three ear bones, but reptiles such as this T. rex don’t. Where did ear bones come from?

Evolution often results in the co-option of a preexisting structure for a new use. Cite three examples from this chapter to support this statement. Evolution can also result in the loss of a trait. Cite three examples from this chapter.

In 1981, a stray black cat with unusual rounded, curled-back ears was adopted by a family in California. Hundreds of descendants of the cat have since been born, and cat fanciers hope to develop the curl cat into a show breed. Suppose you owned the first curl cat and wanted to develop a true-breeding variety. How would you determine whether the curl allele is dominant or recessive? How would you obtain true-breeding curl cats? How could you be sure they are true-breeding?

Testcross Explained

 The parent with the dominant phenotype was heterozygous. 

 No. If the parent had been heterozygous, half of the progeny would have had the dominant phenotype and the other half would have had the recessive phenotype. 

 No. Only a single gene pair is involved. 

 No. The dominant allele is completely dominant. 

 No. If this were correct, then the cross would not be a testcross. 

 The parent with the dominant phenotype was homozygous. 

Try using a Punnett square.

What is indicated when a single-character testcross yields offspring that all have the dominant phenotype?

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[BOOK BIO] CLUTCH

Pedigrees

Genotype vs. Phenotype

Mendel's Laws

X-Inactivation

Incomplete Dominance vs. Codominance

Monohybrid Crosses

Autosomal Inheritance

Non-Mendelian Genetics

Mendel's Experiments

Epistasis

Dihybrid Crosses

Sex-Linked Inheritance

Introduction to Mendel's Experiments

Punnett Square Probability

Punnett Squares

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