Question

in roses, red flowers and long stems are dominant traits. a rose plant that is homozygous for both red flowers and long stems is crossed with a rose plant that is heterozygous for red flowers and homozygous for short stems. what percentage of the offspring will exhibit red, long - stemmed flowers?
25 percent
50 percent
75 percent
100 percent

Explanation:

Step1: Define alleles

Let \( R \) = red (dominant), \( r \) = non - red (recessive); \( L \) = long (dominant), \( l \) = short (recessive).
First parent: homozygous red (\( RR \)) and homozygous long (\( LL \)), so genotype \( RRLL \). Gametes: \( RL \) (only one type, since it's homozygous for both).
Second parent: heterozygous red (\( Rr \)) and homozygous short (\( ll \)), so genotype \( Rrll \). Gametes: \( Rl \) and \( rl \) (from \( Rr \) and \( ll \), since \( ll \) gives only \( l \), and \( Rr \) gives \( R \) and \( r \)).

Step2: Do the Punnett square

The cross is \( RRLL\times Rrll \). But we can also analyze the two traits separately (since they are independent, by Mendel's law of independent assortment) and then multiply the probabilities.

• For flower color: \( RR\times Rr \). The possible genotypes of offspring: \( RR \) and \( Rr \). All offspring will have at least one \( R \) allele, so the probability of red flowers (\( R\_ \)) is \( 100\% \) or \( 1 \).
• For stem length: \( LL\times ll \). The genotype of offspring is \( Ll \). Since \( L \) is dominant, the probability of long stems (\( L\_ \)) is \( 100\% \) or \( 1 \).

Step3: Calculate the combined probability

The probability of having red and long - stemmed flowers is the product of the probability of red flowers and long stems. Since \( P(red)=1 \) and \( P(long)=1 \), \( P(red\cap long)=1\times1 = 1 = 100\% \). Wait, but wait, maybe I made a mistake in the second parent's stem length. Wait, the second parent is homozygous for short stems, so \( ll \), and the first parent is \( LL \). So for stem length: \( LL\times ll \) gives all \( Ll \), which is long (dominant). For flower color: first parent \( RR \), second \( Rr \). \( RR\times Rr \): the cross gives \( RR \) and \( Rr \), both red. So all offspring will have red (because at least one \( R \)) and long (because \( Ll \) has dominant \( L \)). So the percentage of offspring with red, long - stemmed flowers is \( 100\% \). Wait, but let's re - check the second parent's genotype. The problem says "heterozygous for red flowers and homozygous for short stems". So flower color: \( Rr \), stem length: \( ll \). First parent: \( RR \) (homozygous red) and \( LL \) (homozygous long). So gametes of first parent: \( RL \). Gametes of second parent: from \( Rr \): \( R \) and \( r \); from \( ll \): \( l \). So gametes are \( Rl \) and \( rl \). Now, when we do the cross:

• For flower color: \( R \) (from first parent) with \( R \) or \( r \) (from second parent). So all offspring have \( R \) (either \( RR \) or \( Rr \)), so red.
• For stem length: \( L \) (from first parent) with \( l \) (from second parent), so all offspring have \( Ll \), which is long (since \( L \) is dominant). So all offspring (100%) will have red, long - stemmed flowers.

Answer:

100 percent